* [PATCH tip/core/rcu 0/14] Documentation updates for 4.6
@ 2016-02-24 5:00 Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
` (13 more replies)
0 siblings, 14 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani
Hello!
This series contains documentation updates as follows:
1. Update the requirements design-level documentation to record RCU's
responsibility to avoid injecting OS jitter onto nohz_full CPUs.
2. Fix control-dependency summary of memory-barriers.txt to
note that barrier() does not always prevent code reordering
when both legs of an "if" statement begin with identical code.
3. Fix memory-barriers.txt section references.
4. Update the requirements design-level documentation to add
synchronize_rcu_mult().
5. Remove obsolete references to RCU-protected array indexes from
memory-barriers.txt.
6. Update memory-barriers.txt to explicitly state that
rcu_dereference() orders writes to as well as reads from
the RCU-protected data item.
7. Update memory-barriers.txt to distinguish between local and
global transitivity.
8. Update memory-barriers.txt to add alternative outcome from
release-acquire chain.
9. Add design-level documentation for RCU's major data structures.
10. Update the requirements design-level documentation to explain
why rcu_read_lock() needs no barrier() for PREEMPT=n.
11. Remove misleading statement that transitivity is cumulativity
from memory-barriers.txt.
12. Update the requirements design-level documentation to explicitly
state that it is illegal to invoke call_rcu() from an offline CPU.
13. Add text to the design-level documentation for RCU's major data
structures stating how the combining tree keeps lock contention
under control.
14. Clarify compiler store-fusion example in memory-barriers.txt.
Thanx, Paul
------------------------------------------------------------------------
RCU/Design/Data-Structures/BigTreeClassicRCU.svg | 474 ++++
RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg | 499 ++++
RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg | 695 +++++
RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg | 741 ++++++
RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg | 858 +++++++
RCU/Design/Data-Structures/Data-Structures.html | 1395 ++++++++++++
RCU/Design/Data-Structures/Data-Structures.htmlx | 1295 +++++++++++
RCU/Design/Data-Structures/HugeTreeClassicRCU.svg | 939 ++++++++
RCU/Design/Data-Structures/TreeLevel.svg | 828 +++++++
RCU/Design/Data-Structures/TreeMapping.svg | 305 ++
RCU/Design/Data-Structures/TreeMappingLevel.svg | 380 +++
RCU/Design/Data-Structures/blkd_task.svg | 843 +++++++
RCU/Design/Data-Structures/nxtlist.svg | 396 +++
RCU/Design/Requirements/Requirements.html | 235 +-
RCU/Design/Requirements/Requirements.htmlx | 121 +
memory-barriers.txt | 141 -
16 files changed, 10074 insertions(+), 71 deletions(-)
^ permalink raw reply [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
` (12 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
This commit records RCU's responsibility to avoid degrading latencies
of CPUs running tight loops within properly configured workloads,
both in kernel and in userspace.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/Design/Requirements/Requirements.html | 10 +++++++++-
Documentation/RCU/Design/Requirements/Requirements.htmlx | 8 ++++++++
2 files changed, 17 insertions(+), 1 deletion(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index a725f9900ec8..3004baa71bcc 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1,5 +1,5 @@
<!-- DO NOT HAND EDIT. -->
-<!-- Instead, edit Documentation/RCU/Design/Requirements/Requirements.htmlx and run 'sh htmlqqz.sh Documentation/RCU/Design/Requirements/Requirements' -->
+<!-- Instead, edit Requirements.htmlx and run 'sh htmlqqz.sh Requirements' -->
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
@@ -2171,6 +2171,14 @@ This real-time requirement motivated the grace-period kthread, which
also simplified handling of a number of race conditions.
<p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
+<p>
Finally, RCU's status as a synchronization primitive means that
any RCU failure can result in arbitrary memory corruption that can be
extremely difficult to debug.
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 3a97ba490c42..61caffc86823 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -2338,6 +2338,14 @@ This real-time requirement motivated the grace-period kthread, which
also simplified handling of a number of race conditions.
<p>
+RCU must avoid degrading real-time response for CPU-bound threads, whether
+executing in usermode (which is one use case for
+<tt>CONFIG_NO_HZ_FULL=y</tt>) or in the kernel.
+That said, CPU-bound loops in the kernel must execute
+<tt>cond_resched_rcu_qs()</tt> at least once per few tens of milliseconds
+in order to avoid receiving an IPI from RCU.
+
+<p>
Finally, RCU's status as a synchronization primitive means that
any RCU failure can result in arbitrary memory corruption that can be
extremely difficult to debug.
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 21:12 ` Mathieu Desnoyers
2016-02-24 5:00 ` [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references Paul E. McKenney
` (11 subsequent siblings)
13 siblings, 1 reply; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
barrier() may be used to prevent compiler reordering when more than one
leg of the control-dependent "if" statement start with identical stores.
This is incorrect at high optimization levels. This commit therefore
updates the summary to match the detailed description.
Reported by: Jianyu Zhan <nasa4836@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 904ee42d078e..e26058d3e253 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -800,9 +800,13 @@ In summary:
use smp_rmb(), smp_wmb(), or, in the case of prior stores and
later loads, smp_mb().
- (*) If both legs of the "if" statement begin with identical stores
- to the same variable, a barrier() statement is required at the
- beginning of each leg of the "if" statement.
+ (*) If both legs of the "if" statement begin with identical stores to
+ the same variable, then those stores must be ordered, either by
+ preceding both of them with smp_mb() or by using smp_store_release()
+ to carry out the stores. Please note that it is -not- sufficient
+ to use barrier() at beginning of each leg of the "if" statement,
+ as optimizing compilers do not necessarily respect barrier()
+ in this case.
(*) Control dependencies require at least one run-time conditional
between the prior load and the subsequent store, and this
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements Paul E. McKenney
` (10 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
This commit fixes a couple of "Compiler Barrier" section references to
be "COMPILER BARRIER". This makes it easier to find the section in
the usual text editors.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index e26058d3e253..c90922b9b294 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -232,7 +232,7 @@ And there are a number of things that _must_ or _must_not_ be assumed:
with memory references that are not protected by READ_ONCE() and
WRITE_ONCE(). Without them, the compiler is within its rights to
do all sorts of "creative" transformations, which are covered in
- the Compiler Barrier section.
+ the COMPILER BARRIER section.
(*) It _must_not_ be assumed that independent loads and stores will be issued
in the order given. This means that for:
@@ -818,7 +818,7 @@ In summary:
(*) Control dependencies require that the compiler avoid reordering the
dependency into nonexistence. Careful use of READ_ONCE() or
atomic{,64}_read() can help to preserve your control dependency.
- Please see the Compiler Barrier section for more information.
+ Please see the COMPILER BARRIER section for more information.
(*) Control dependencies pair normally with other types of barriers.
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (2 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes Paul E. McKenney
` (9 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
.../RCU/Design/Requirements/Requirements.html | 92 ++++++++++++++++++++++
.../RCU/Design/Requirements/Requirements.htmlx | 82 +++++++++++++++++++
2 files changed, 174 insertions(+)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 3004baa71bcc..59acd82e67d4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -2231,6 +2231,8 @@ described in a separate section.
<li> <a href="#Sched Flavor">Sched Flavor</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
+<li> <a href="#Waiting for Multiple Grace Periods">
+ Waiting for Multiple Grace Periods</a>
</ol>
<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
@@ -2480,6 +2482,81 @@ The tasks-RCU API is quite compact, consisting only of
<tt>synchronize_rcu_tasks()</tt>, and
<tt>rcu_barrier_tasks()</tt>.
+<h3><a name="Waiting for Multiple Grace Periods">
+Waiting for Multiple Grace Periods</a></h3>
+
+<p>
+Perhaps you have an RCU protected data structure that is accessed from
+RCU read-side critical sections, from softirq handlers, and from
+hardware interrupt handlers.
+That is three flavors of RCU, the normal flavor, the bottom-half flavor,
+and the sched flavor.
+How to wait for a compound grace period?
+
+<p>
+The best approach is usually to “just say no!” and
+insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+around each RCU read-side critical section, regardless of what
+environment it happens to be in.
+But suppose that some of the RCU read-side critical sections are
+on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
+is not a viable option, so that <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are not free.
+What then?
+
+<p>
+You <i>could</i> wait on all three grace periods in succession, as follows:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+</pre>
+</blockquote>
+
+<p>
+This works, but triples the update-side latency penalty.
+In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
+may be used to wait on all three flavors of grace period concurrently:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+</pre>
+</blockquote>
+
+<p>
+But what if it is necessary to also wait on SRCU?
+This can be done as follows:
+
+<blockquote>
+<pre>
+ 1 static void call_my_srcu(struct rcu_head *head,
+ 2 void (*func)(struct rcu_head *head))
+ 3 {
+ 4 call_srcu(&my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+</pre>
+</blockquote>
+
+<p>
+If you needed to wait on multiple different flavors of SRCU
+(but why???), you would need to create a wrapper function resembling
+<tt>call_my_srcu()</tt> for each SRCU flavor.
+
+<p><a name="Quick Quiz 15"><b>Quick Quiz 15</b>:</a>
+But what if I need to wait for multiple RCU flavors, but I also need
+the grace periods to be expedited?
+<br><a href="#qq15answer">Answer</a>
+
+<p>
+Again, it is usually better to adjust the RCU read-side critical sections
+to use a single flavor of RCU, but when this is not feasible, you can use
+<tt>synchronize_rcu_mult()</tt>.
+
<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
<p>
@@ -2901,5 +2978,20 @@ during scheduler initialization.
</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
+<a name="qq15answer"></a>
+<p><b>Quick Quiz 15</b>:
+But what if I need to wait for multiple RCU flavors, but I also need
+the grace periods to be expedited?
+
+
+</p><p><b>Answer</b>:
+If you are using expedited grace periods, there should be less penalty
+for waiting on them in succession.
+But if that is nevertheless a problem, you can use workqueues or multiple
+kthreads to wait on the various expedited grace periods concurrently.
+
+
+</p><p><a href="#Quick%20Quiz%2015"><b>Back to Quick Quiz 15</b>.</a>
+
</body></html>
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 61caffc86823..6ff4966672e2 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -2398,6 +2398,8 @@ described in a separate section.
<li> <a href="#Sched Flavor">Sched Flavor</a>
<li> <a href="#Sleepable RCU">Sleepable RCU</a>
<li> <a href="#Tasks RCU">Tasks RCU</a>
+<li> <a href="#Waiting for Multiple Grace Periods">
+ Waiting for Multiple Grace Periods</a>
</ol>
<h3><a name="Bottom-Half Flavor">Bottom-Half Flavor</a></h3>
@@ -2647,6 +2649,86 @@ The tasks-RCU API is quite compact, consisting only of
<tt>synchronize_rcu_tasks()</tt>, and
<tt>rcu_barrier_tasks()</tt>.
+<h3><a name="Waiting for Multiple Grace Periods">
+Waiting for Multiple Grace Periods</a></h3>
+
+<p>
+Perhaps you have an RCU protected data structure that is accessed from
+RCU read-side critical sections, from softirq handlers, and from
+hardware interrupt handlers.
+That is three flavors of RCU, the normal flavor, the bottom-half flavor,
+and the sched flavor.
+How to wait for a compound grace period?
+
+<p>
+The best approach is usually to “just say no!” and
+insert <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+around each RCU read-side critical section, regardless of what
+environment it happens to be in.
+But suppose that some of the RCU read-side critical sections are
+on extremely hot code paths, and that use of <tt>CONFIG_PREEMPT=n</tt>
+is not a viable option, so that <tt>rcu_read_lock()</tt> and
+<tt>rcu_read_unlock()</tt> are not free.
+What then?
+
+<p>
+You <i>could</i> wait on all three grace periods in succession, as follows:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu();
+ 2 synchronize_rcu_bh();
+ 3 synchronize_sched();
+</pre>
+</blockquote>
+
+<p>
+This works, but triples the update-side latency penalty.
+In cases where this is not acceptable, <tt>synchronize_rcu_mult()</tt>
+may be used to wait on all three flavors of grace period concurrently:
+
+<blockquote>
+<pre>
+ 1 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched);
+</pre>
+</blockquote>
+
+<p>
+But what if it is necessary to also wait on SRCU?
+This can be done as follows:
+
+<blockquote>
+<pre>
+ 1 static void call_my_srcu(struct rcu_head *head,
+ 2 void (*func)(struct rcu_head *head))
+ 3 {
+ 4 call_srcu(&my_srcu, head, func);
+ 5 }
+ 6
+ 7 synchronize_rcu_mult(call_rcu, call_rcu_bh, call_rcu_sched, call_my_srcu);
+</pre>
+</blockquote>
+
+<p>
+If you needed to wait on multiple different flavors of SRCU
+(but why???), you would need to create a wrapper function resembling
+<tt>call_my_srcu()</tt> for each SRCU flavor.
+
+<p>@@QQ@@
+But what if I need to wait for multiple RCU flavors, but I also need
+the grace periods to be expedited?
+<p>@@QQA@@
+If you are using expedited grace periods, there should be less penalty
+for waiting on them in succession.
+But if that is nevertheless a problem, you can use workqueues or multiple
+kthreads to wait on the various expedited grace periods concurrently.
+<p>@@QQE@@
+
+<p>
+Again, it is usually better to adjust the RCU read-side critical sections
+to use a single flavor of RCU, but when this is not feasible, you can use
+<tt>synchronize_rcu_mult()</tt>.
+
<h2><a name="Possible Future Changes">Possible Future Changes</a></h2>
<p>
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (3 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() Paul E. McKenney
` (8 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
Commit #1ebee8017d84 (rcu: Eliminate array-index-based RCU primitives)
eliminated the primitives supporting RCU-protected array indexes, but
failed to update Documentation/memory-barriers.txt accordingly. This
commit therefore removes the discussion of RCU-protected array indexes.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 15 ---------------
1 file changed, 15 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index c90922b9b294..6bee0a2c43ab 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -565,21 +565,6 @@ odd-numbered bank is idle, one can see the new value of the pointer P (&B),
but the old value of the variable B (2).
-Another example of where data dependency barriers might be required is where a
-number is read from memory and then used to calculate the index for an array
-access:
-
- CPU 1 CPU 2
- =============== ===============
- { M[0] == 1, M[1] == 2, M[3] = 3, P == 0, Q == 3 }
- M[1] = 4;
- <write barrier>
- WRITE_ONCE(P, 1);
- Q = READ_ONCE(P);
- <data dependency barrier>
- D = M[Q];
-
-
The data dependency barrier is very important to the RCU system,
for example. See rcu_assign_pointer() and rcu_dereference() in
include/linux/rcupdate.h. This permits the current target of an RCU'd
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference()
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (4 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity Paul E. McKenney
` (7 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
The current memory-barriers.txt does not address the possibility of
a write to a dereferenced pointer. This should be rare, but when it
happens, we need that write -not- to be clobbered by the initialization.
This commit therefore adds an example showing a data dependency ordering
a later data-dependent write.
Reported-by: Leonid Yegoshin <Leonid.Yegoshin@imgtec.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 24 ++++++++++++++++++++++++
1 file changed, 24 insertions(+)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 6bee0a2c43ab..e9ebeb3b1077 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -555,6 +555,30 @@ between the address load and the data load:
This enforces the occurrence of one of the two implications, and prevents the
third possibility from arising.
+A data-dependency barrier must also order against dependent writes:
+
+ CPU 1 CPU 2
+ =============== ===============
+ { A == 1, B == 2, C = 3, P == &A, Q == &C }
+ B = 4;
+ <write barrier>
+ WRITE_ONCE(P, &B);
+ Q = READ_ONCE(P);
+ <data dependency barrier>
+ *Q = 5;
+
+The data-dependency barrier must order the read into Q with the store
+into *Q. This prohibits this outcome:
+
+ (Q == B) && (B == 4)
+
+Please note that this pattern should be rare. After all, the whole point
+of dependency ordering is to -prevent- writes to the data structure, along
+with the expensive cache misses associated with those writes. This pattern
+can be used to record rare error conditions and the like, and the ordering
+prevents such records from being lost.
+
+
[!] Note that this extremely counterintuitive situation arises most easily on
machines with split caches, so that, for example, one cache bank processes
even-numbered cache lines and the other bank processes odd-numbered cache
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (5 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome Paul E. McKenney
` (6 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
The introduction of smp_load_acquire() and smp_store_release() had
the side effect of introducing a weaker notion of transitivity:
The transitivity of full smp_mb() barriers is global, but that
of smp_store_release()/smp_load_acquire() chains is local. This
commit therefore introduces the notion of local transitivity and
gives an example.
Reported-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 78 ++++++++++++++++++++++++++++++++++++++-
1 file changed, 76 insertions(+), 2 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index e9ebeb3b1077..ae9d306725ba 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1318,8 +1318,82 @@ or a level of cache, CPU 2 might have early access to CPU 1's writes.
General barriers are therefore required to ensure that all CPUs agree
on the combined order of CPU 1's and CPU 2's accesses.
-To reiterate, if your code requires transitivity, use general barriers
-throughout.
+General barriers provide "global transitivity", so that all CPUs will
+agree on the order of operations. In contrast, a chain of release-acquire
+pairs provides only "local transitivity", so that only those CPUs on
+the chain are guaranteed to agree on the combined order of the accesses.
+For example, switching to C code in deference to Herman Hollerith:
+
+ int u, v, x, y, z;
+
+ void cpu0(void)
+ {
+ r0 = smp_load_acquire(&x);
+ WRITE_ONCE(u, 1);
+ smp_store_release(&y, 1);
+ }
+
+ void cpu1(void)
+ {
+ r1 = smp_load_acquire(&y);
+ r4 = READ_ONCE(v);
+ r5 = READ_ONCE(u);
+ smp_store_release(&z, 1);
+ }
+
+ void cpu2(void)
+ {
+ r2 = smp_load_acquire(&z);
+ smp_store_release(&x, 1);
+ }
+
+ void cpu3(void)
+ {
+ WRITE_ONCE(v, 1);
+ smp_mb();
+ r3 = READ_ONCE(u);
+ }
+
+Because cpu0(), cpu1(), and cpu2() participate in a local transitive
+chain of smp_store_release()/smp_load_acquire() pairs, the following
+outcome is prohibited:
+
+ r0 == 1 && r1 == 1 && r2 == 1
+
+Furthermore, because of the release-acquire relationship between cpu0()
+and cpu1(), cpu1() must see cpu0()'s writes, so that the following
+outcome is prohibited:
+
+ r1 == 1 && r5 == 0
+
+However, the transitivity of release-acquire is local to the participating
+CPUs and does not apply to cpu3(). Therefore, the following outcome
+is possible:
+
+ r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
+
+Although cpu0(), cpu1(), and cpu2() will see their respective reads and
+writes in order, CPUs not involved in the release-acquire chain might
+well disagree on the order. This disagreement stems from the fact that
+the weak memory-barrier instructions used to implement smp_load_acquire()
+and smp_store_release() are not required to order prior stores against
+subsequent loads in all cases. This means that cpu3() can see cpu0()'s
+store to u as happening -after- cpu1()'s load from v, even though
+both cpu0() and cpu1() agree that these two operations occurred in the
+intended order.
+
+However, please keep in mind that smp_load_acquire() is not magic.
+In particular, it simply reads from its argument with ordering. It does
+-not- ensure that any particular value will be read. Therefore, the
+following outcome is possible:
+
+ r0 == 0 && r1 == 0 && r2 == 0 && r5 == 0
+
+Note that this outcome can happen even on a mythical sequentially
+consistent system where nothing is ever reordered.
+
+To reiterate, if your code requires global transitivity, use general
+barriers throughout.
========================
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (6 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures Paul E. McKenney
` (5 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
The memory-barriers.txt discussion of local transitivity and
release-acquire chains leaves out discussion of the outcome of
the read from "u". This commit therefore adds an outcome showing
that you can get a "1" from this read even if the release-acquire
pairs don't line up.
Reported-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 4 ++++
1 file changed, 4 insertions(+)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index ae9d306725ba..57e4a4b053c5 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1372,6 +1372,10 @@ is possible:
r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0
+As an aside, the following outcome is also possible:
+
+ r0 == 0 && r1 == 1 && r2 == 1 && r3 == 0 && r4 == 0 && r5 == 1
+
Although cpu0(), cpu1(), and cpu2() will see their respective reads and
writes in order, CPUs not involved in the release-acquire chain might
well disagree on the order. This disagreement stems from the fact that
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (7 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() Paul E. McKenney
` (4 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
This commit adds documentation for RCU's major data structures,
including rcu_state, rcu_node, rcu_data, rcu_dynticks, and rcu_head.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
.../Design/Data-Structures/BigTreeClassicRCU.svg | 474 +++++++
.../Design/Data-Structures/BigTreeClassicRCUBH.svg | 499 +++++++
.../Data-Structures/BigTreeClassicRCUBHdyntick.svg | 695 ++++++++++
.../Data-Structures/BigTreePreemptRCUBHdyntick.svg | 741 +++++++++++
.../BigTreePreemptRCUBHdyntickCB.svg | 858 ++++++++++++
.../Design/Data-Structures/Data-Structures.html | 1372 ++++++++++++++++++++
.../Design/Data-Structures/Data-Structures.htmlx | 1272 ++++++++++++++++++
.../Design/Data-Structures/HugeTreeClassicRCU.svg | 939 ++++++++++++++
.../RCU/Design/Data-Structures/TreeLevel.svg | 828 ++++++++++++
.../RCU/Design/Data-Structures/TreeMapping.svg | 305 +++++
.../Design/Data-Structures/TreeMappingLevel.svg | 380 ++++++
.../RCU/Design/Data-Structures/blkd_task.svg | 843 ++++++++++++
.../RCU/Design/Data-Structures/nxtlist.svg | 396 ++++++
13 files changed, 9602 insertions(+)
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create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBH.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreeClassicRCUBHdyntick.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntick.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/BigTreePreemptRCUBHdyntickCB.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/Data-Structures.html
create mode 100644 Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx
create mode 100644 Documentation/RCU/Design/Data-Structures/HugeTreeClassicRCU.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/TreeLevel.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/TreeMapping.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/TreeMappingLevel.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/blkd_task.svg
create mode 100644 Documentation/RCU/Design/Data-Structures/nxtlist.svg
diff --git a/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg
new file mode 100644
index 000000000000..727e270b11e4
--- /dev/null
+++ b/Documentation/RCU/Design/Data-Structures/BigTreeClassicRCU.svg
@@ -0,0 +1,474 @@
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+ font-size="288"
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+ id="text58">struct</text>
+ <!-- Text -->
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+ xml:space="preserve"
+ x="1575"
+ y="9675"
+ fill="#000000"
+ font-family="Courier"
+ font-style="normal"
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+ font-size="288"
+ text-anchor="middle"
+ id="text60">rcu_data</text>
+ <!-- Text -->
+ <text
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+ y="10350"
+ fill="#000000"
+ font-family="Helvetica"
+ font-style="normal"
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+ font-style="normal"
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+ font-size="288"
+ text-anchor="middle"
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+ <!-- Text -->
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+ xml:space="preserve"
+ x="3375"
+ y="8100"
+ fill="#000000"
+ font-family="Helvetica"
+ font-style="normal"
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+ font-size="324"
+ text-anchor="middle"
+ id="text68">CPU 15</text>
+ <!-- Text -->
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+ y="9225"
+ fill="#000000"
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+ font-style="normal"
+ font-weight="bold"
+ font-size="288"
+ text-anchor="middle"
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+ <!-- Text -->
+ <text
+ xml:space="preserve"
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+ y="9675"
+ fill="#000000"
+ font-family="Courier"
+ font-style="normal"
+ font-weight="bold"
+ font-size="288"
+ text-anchor="middle"
+ id="text72">rcu_data</text>
+ <!-- Text -->
+ <text
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+ x="6975"
+ y="10350"
+ fill="#000000"
+ font-family="Helvetica"
+ font-style="normal"
+ font-weight="normal"
+ font-size="324"
+ text-anchor="middle"
+ id="text74">CPU 1007</text>
+ <!-- Text -->
+ <text
+ xml:space="preserve"
+ x="8730"
+ y="6930"
+ fill="#000000"
+ font-family="Courier"
+ font-style="normal"
+ font-weight="bold"
+ font-size="288"
+ text-anchor="middle"
+ id="text76">struct</text>
+ <!-- Text -->
+ <text
+ xml:space="preserve"
+ x="8730"
+ y="7380"
+ fill="#000000"
+ font-family="Courier"
+ font-style="normal"
+ font-weight="bold"
+ font-size="288"
+ text-anchor="middle"
+ id="text78">rcu_data</text>
+ <!-- Text -->
+ <text
+ xml:space="preserve"
+ x="8730"
+ y="8055"
+ fill="#000000"
+ font-family="Helvetica"
+ font-style="normal"
+ font-weight="normal"
+ font-size="324"
+ text-anchor="middle"
+ id="text80">CPU 1023</text>
+ <!-- Text -->
+ <text
+ xml:space="preserve"
+ x="225"
+ y="450"
+ fill="#000000"
+ font-family="Courier"
+ font-style="normal"
+ font-weight="bold"
+ font-size="288"
+ text-anchor="start"
+ id="text82">struct rcu_state</text>
+ <!-- Text -->
+ <text
+ xml:space="preserve"
+ x="2475"
+ y="4050"
+ fill="#000000"
+ font-family="Courier"
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diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
new file mode 100644
index 000000000000..ba9fbb5177f6
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+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -0,0 +1,1372 @@
+<!-- DO NOT HAND EDIT. -->
+<!-- Instead, edit Data-Structures.htmlx and run 'sh htmlqqz.sh Data-Structures' -->
+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+ "http://www.w3.org/TR/html4/loose.dtd">
+ <html>
+ <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
+ <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+ <p>January 27, 2016</p>
+ <p>This article was contributed by Paul E. McKenney</p>
+
+<h3>Introduction</h3>
+
+This document describes RCU's major data structures and their relationship
+to each other.
+
+<ol>
+<li> <a href="#Data-Structure Relationships">
+ Data-Structure Relationships</a>
+<li> <a href="#The rcu_state Structure">
+ The <tt>rcu_state</tt> Structure</a>
+<li> <a href="#The rcu_node Structure">
+ The <tt>rcu_node</tt> Structure</a>
+<li> <a href="#The rcu_data Structure">
+ The <tt>rcu_data</tt> Structure</a>
+<li> <a href="#The rcu_dynticks Structure">
+ The <tt>rcu_dynticks</tt> Structure</a>
+<li> <a href="#The rcu_head Structure">
+ The <tt>rcu_head</tt> Structure</a>
+<li> <a href="#RCU-Specific Fields in the task_struct Structure">
+ RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
+<li> <a href="#Accessor Functions">
+ Accessor Functions</a>
+</ol>
+
+At the end we have the
+<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+
+<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
+
+<p>RCU is for all intents and purposes a large state machine, and its
+data structures maintain the state in such a way as to allow RCU readers
+to execute extremely quickly, while also processing the RCU grace periods
+requested by updaters in an efficient and extremely scalable fashion.
+The efficiency and scalability of RCU updaters is provided primarily
+by a combining tree, as shown below:
+
+</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
+
+</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
+containing a tree of <tt>rcu_node</tt> structures.
+Each leaf node of the <tt>rcu_node</tt> tree has up to 16
+<tt>rcu_data</tt> structures associated with it, so that there
+are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
+one for each possible CPU.
+This structure is adjusted at boot time, if needed, to handle the
+common case where <tt>nr_cpu_ids</tt> is much less than
+<tt>NR_CPUs</tt>.
+For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
+which results in a three-level <tt>rcu_node</tt> tree.
+If the actual hardware has only 16 CPUs, RCU will adjust itself
+at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
+
+</p><p>The purpose of this combining tree is to allow per-CPU events
+such as quiescent states, dyntick-idle transitions,
+and CPU hotplug operations to be processed efficiently
+and scalably.
+Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
+and other events are recorded by the leaf-level <tt>rcu_node</tt>
+structures.
+All of these events are combined at each level of the tree until finally
+grace periods are completed at the tree's root <tt>rcu_node</tt>
+structure.
+A grace period can be completed at the root once every CPU
+(or, in the case of <tt>CONFIG_TREE_PREEMPT_RCU</tt>, task)
+has passed through a quiescent state.
+Once a grace period has completed, record of that fact is propagated
+back down the tree.
+
+</p><p>As can be seen from the diagram, on a 64-bit system
+a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
+of 64 at the root and a fanout of 16 at the leaves.
+
+<p><a name="Quick Quiz 1"><b>Quick Quiz 1</b>:</a>
+Why isn't the fanout at the leaves also 64?
+<br><a href="#qq1answer">Answer</a>
+
+</p><p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
+a 32-bit system), then RCU will automatically add more levels to the
+tree.
+For example, if you are crazy enough to build a 64-bit system with 65,536
+CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
+
+</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
+
+</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
+accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
+32-bit systems.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
+as small as 2 if you wish, which would permit only 16 CPUs, which
+is useful for testing.
+
+</p><p>The Linux kernel actually supports multiple flavors of RCU
+running concurrently, so RCU builds separate data structures for each
+flavor.
+For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
+rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
+
+</p><p>Energy efficiency is increasingly important, and for that
+reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
+turns off the scheduling-clock interrupts on idle CPUs, which in
+turn allows those CPUs to attain deeper sleep states and to consume
+less energy.
+CPUs whose scheduling-clock interrupts have been turned off are
+said to be in <i>dyntick-idle mode</i>.
+RCU must handle dyntick-idle CPUs specially
+because RCU would otherwise wake up each CPU on every grace period,
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
+RCU uses the <tt>rcu_dynticks</tt> structure to track
+which CPUs are in dyntick idle mode, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
+
+</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
+for all flavors of RCU.
+Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
+CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
+that <tt>rcu_dynticks</tt>, as shown in the figure.
+
+</p><p>Kernels built with <tt>CONFIG_TREE_PREEMPT_RCU</tt> support
+rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
+
+</p><p>RCU updaters wait for normal grace periods by registering
+RCU callbacks, either directly via <tt>call_rcu()</tt> and
+friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+there being a separate interface per flavor of RCU)
+or indirectly via <tt>synchronize_rcu()</tt> and friends.
+RCU callbacks are represented by <tt>rcu_head</tt> structures,
+which are queued on <tt>rcu_data</tt> structures while they are
+waiting for a grace period to elapse, as shown in the following figure:
+
+</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
+
+</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
+<tt>TREE_PREEMPT_RCU</tt>'s major data structures are related.
+Lesser data structures will be introduced with the algorithms that
+make use of them.
+
+</p><p>Note that each of the data structures in the above figure has
+its own synchronization:
+
+<p><ol>
+<li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
+ and some fields are protected by the corresponding root
+ <tt>rcu_node</tt> structure's lock.
+<li> Each <tt>rcu_node</tt> structure has a spinlock.
+<li> The fields in <tt>rcu_data</tt> are private to the corresponding
+ CPU, although a few can be read and written by other CPUs.
+<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private
+ to the corresponding CPU, although a few can be read by
+ other CPUs.
+</ol>
+
+<p>It is important to note that different data structures can have
+very different ideas about the state of RCU at any given time.
+For but one example, awareness of the start or end of a given RCU
+grace period propagates slowly through the data structures.
+This slow propagation is absolutely necessary for RCU to have good
+read-side performance.
+If this balkanized implementation seems foreign to you, one useful
+trick is to consider each instance of these data structures to be
+a different person, each having the usual slightly different
+view of reality.
+
+</p><p>The general role of each of these data structures is as
+follows:
+
+</p><ol>
+<li> <tt>rcu_state</tt>:
+ This structure forms the interconnection between the
+ <tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+ tracks grace periods, serves as short-term repository
+ for callbacks orphaned by CPU-hotplug events,
+ maintains <tt>rcu_barrier()</tt> state,
+ tracks expedited grace-period state,
+ and maintains state used to force quiescent states when
+ grace periods extend too long,
+<li> <tt>rcu_node</tt>: This structure forms the combining
+ tree that propagates quiescent-state
+ information from the leaves to the root, and also propagates
+ grace-period information from the root to the leaves.
+ It provides local copies of the grace-period state in order
+ to allow this information to be accessed in a synchronized
+ manner without suffering the scalability limitations that
+ would otherwise be imposed by global locking.
+ In <tt>CONFIG_TREE_PREEMPT_RCU</tt> kernels, it manages the lists
+ of tasks that have blocked while in their current
+ RCU read-side critical section.
+ In <tt>CONFIG_TREE_PREEMPT_RCU</tt> with
+ <tt>CONFIG_RCU_BOOST</tt>, it manages the
+ per-<tt>rcu_node</tt> priority-boosting
+ kernel threads (kthreads) and state.
+ Finally, it records CPU-hotplug state in order to determine
+ which CPUs should be ignored during a given grace period.
+<li> <tt>rcu_data</tt>: This per-CPU structure is the
+ focus of quiescent-state detection and RCU callback queuing.
+ It also tracks its relationship to the corresponding leaf
+ <tt>rcu_node</tt> structure to allow more-efficient
+ propagation of quiescent states up the <tt>rcu_node</tt>
+ combining tree.
+ Like the <tt>rcu_node</tt> structure, it provides a local
+ copy of the grace-period information to allow for-free
+ synchronized
+ access to this information from the corresponding CPU.
+ Finally, this structure records past dyntick-idle state
+ for the corresponding CPU and also tracks statistics.
+<li> <tt>rcu_dynticks</tt>:
+ This per-CPU structure tracks the current dyntick-idle
+ state for the corresponding CPU.
+ Unlike the other three structures, the <tt>rcu_dynticks</tt>
+ structure is not replicated per RCU flavor.
+<li> <tt>rcu_head</tt>:
+ This structure represents RCU callbacks, and is the
+ only structure allocated and managed by RCU users.
+ The <tt>rcu_head</tt> structure is normally embedded
+ within the RCU-protected data structure.
+</ol>
+
+<p>If all you wanted from this article was a general notion of how
+RCU's data structures are related, you are done.
+Otherwise, each of the following sections give more details on
+the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
+and <tt>rcu_dynticks</tt> data structures.
+
+<h3><a name="The rcu_state Structure">
+The <tt>rcu_state</tt> Structure</a></h3>
+
+<p>The <tt>rcu_state</tt> structure is the base structure that
+represents a flavor of RCU.
+This structure forms the interconnection between the
+<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+tracks grace periods, contains the lock used to
+synchronize with CPU-hotplug events,
+and maintains state used to force quiescent states when
+grace periods extend too long,
+
+</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+The more specialized fields are covered in the discussion of their
+use.
+
+<h5>Relationship to rcu_node and rcu_data Structures</h5>
+
+This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_node node[NUM_RCU_NODES];
+ 2 struct rcu_node *level[NUM_RCU_LVLS + 1];
+ 3 struct rcu_data __percpu *rda;
+</pre>
+
+<p><a name="Quick Quiz 2"><b>Quick Quiz 2</b>:</a>
+Wait a minute!
+You said that the <tt>rcu_node</tt> structures formed a tree,
+but they are declared as a flat array!
+What gives?
+<br><a href="#qq2answer">Answer</a>
+
+</p><p>The <tt>rcu_node</tt> tree is embedded into the
+<tt>->node[]</tt> array as shown in the following figure:
+
+</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
+
+</p><p>One interesting consequence of this mapping is that a
+breadth-first traversal of the tree is implemented as a simple
+linear scan of the array, which is in fact what the
+<tt>rcu_for_each_node_breadth_first()</tt> macro does.
+This macro is used at the beginning and ends of grace periods.
+
+</p><p>Each entry of the <tt>->level</tt> array references
+the first <tt>rcu_node</tt> structure on the corresponding level
+of the tree, for example, as shown below:
+
+</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
+
+</p><p>The zero<sup>th</sup> element of the array references the root
+<tt>rcu_node</tt> structure, the first element references the
+first child of the root <tt>rcu_node</tt>, and finally the second
+element references the first leaf <tt>rcu_node</tt> structure.
+
+<p><a name="Quick Quiz 3"><b>Quick Quiz 3</b>:</a>
+Given that this array represents a tree, why can't the diagram that
+includes the <tt>->level</tt> array be planar?
+<br><a href="#qq3answer">Answer</a>
+
+</p><p>Finally, the <tt>->rda</tt> field references a per-CPU
+pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
+
+</p><p>All of these fields are constant once initialization is complete,
+and therefore need no protection.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gpnum;
+ 2 unsigned long completed;
+</pre>
+
+<p>RCU grace periods are numbered, and
+the <tt>->gpnum</tt> field contains the number of the grace
+period that started most recently.
+The <tt>->completed</tt> field contains the number of the
+grace period that completed most recently.
+If the two fields are equal, the RCU grace period that most recently
+started has already completed, and therefore the corresponding
+flavor of RCU is idle.
+If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>,
+then <tt>->gpnum</tt> gives the number of the current RCU
+grace period, which has not yet completed.
+Any other combination of values indicates that something is broken.
+These two fields are protected by the root <tt>rcu_node</tt>'s
+<tt>->lock</tt> field.
+
+</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields
+in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
+as well.
+The fields in the <tt>rcu_state</tt> structure represent the
+most current values, and those of the other structures are compared
+in order to detect the start of a new grace period in a distributed
+fashion.
+The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
+(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
+
+<h5>Miscellaneous</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gp_max;
+ 2 char abbr;
+ 3 char *name;
+</pre>
+
+<p>The <tt>->gp_max</tt> field tracks the duration of the longest
+grace period in jiffies.
+It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>.
+
+<p>The <tt>->name</tt> field points to the name of the RCU flavor
+(for example, “rcu_sched”), and is constant.
+The <tt>->abbr</tt> field contains a one-character abbreviation,
+for example, “s” for RCU-sched.
+
+<h3><a name="The rcu_node Structure">
+The <tt>rcu_node</tt> Structure</a></h3>
+
+<p>The <tt>rcu_node</tt> structures form the combining
+tree that propagates quiescent-state
+information from the leaves to the root and also that propagates
+grace-period information from the root down to the leaves.
+They provides local copies of the grace-period state in order
+to allow this information to be accessed in a synchronized
+manner without suffering the scalability limitations that
+would otherwise be imposed by global locking.
+In <tt>CONFIG_TREE_PREEMPT_RCU</tt> kernels, they manage the lists
+of tasks that have blocked while in their current
+RCU read-side critical section.
+In <tt>CONFIG_TREE_PREEMPT_RCU</tt> with
+<tt>CONFIG_RCU_BOOST</tt>, they manage the
+per-<tt>rcu_node</tt> priority-boosting
+kernel threads (kthreads) and state.
+Finally, they record CPU-hotplug state in order to determine
+which CPUs should be ignored during a given grace period.
+
+</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Combining Tree</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_node *parent;
+ 2 u8 level;
+ 3 u8 grpnum;
+ 4 unsigned long grpmask;
+ 5 int grplo;
+ 6 int grphi;
+</pre>
+
+<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt>
+one level up in the tree, and is <tt>NULL</tt> for the root
+<tt>rcu_node</tt>.
+The RCU implementation makes heavy use of this field to push quiescent
+states up the tree.
+The <tt>->level</tt> field gives the level in the tree, with
+the root being at level zero, its children at level one, and so on.
+The <tt>->grpnum</tt> field gives this node's position within
+the children of its parent, so this number can range between 0 and 31
+on 32-bit systems and between 0 and 63 on 64-bit systems.
+The <tt>->level</tt> and <tt>->grpnum</tt> fields are
+used only during initialization and for tracing.
+The <tt>->grpmask</tt> field is the bitmask counterpart of
+<tt>->grpnum</tt>, and therefore always has exactly one bit set.
+This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
+structure in its parent's bitmasks, which are described later.
+Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields
+contain the lowest and highest numbered CPU served by this
+<tt>rcu_node</tt> structure, respectively.
+
+</p><p>All of these fields are constant, and thus do not require any
+synchronization.
+
+<h5>Synchronization</h5>
+
+<p>This field of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 raw_spinlock_t lock;
+</pre>
+
+<p>This field is used to protect the remaining fields in this structure,
+unless otherwise stated.
+That said, all of the fields in this structure can be accessed without
+locking for tracing purposes.
+Yes, this can result in confusing traces, but better some tracing confusion
+than to be heisenbugged out of existence.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gpnum;
+ 2 unsigned long completed;
+</pre>
+
+<p>These fields are the counterparts of the fields of the same name in
+the <tt>rcu_state</tt> structure.
+They each may lag up to one behind their <tt>rcu_state</tt>
+counterparts.
+If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and
+<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> structure,
+the <tt>->gpnum</tt> field will be one greater than the
+<tt>->complete</tt> fields, with <tt>->gpnum</tt>
+indicating which grace period this <tt>rcu_node</tt> believes
+is still being waited for.
+
+</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning
+of each grace period, and the <tt>->completed</tt> fields are
+updated at the end of each grace period.
+
+<h5>Quiescent-State Tracking</h5>
+
+<p>These fields manage the propagation of quiescent states up the
+combining tree.
+
+</p><p>This portion of the <tt>rcu_node</tt> structure has fields
+as follows:
+
+<pre>
+ 1 unsigned long qsmask;
+ 2 unsigned long expmask;
+ 3 unsigned long qsmaskinit;
+ 4 unsigned long expmaskinit;
+</pre>
+
+<p>The <tt>->qsmask</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current normal grace period.
+Such children will have a value of 1 in their corresponding bit.
+Note that the leaf <tt>rcu_node</tt> structures should be
+thought of as having <tt>rcu_data</tt> structures as their
+children.
+Similarly, the <tt>->expmask</tt> field tracks which
+of this <tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current expedited grace period.
+An expedited grace period has
+the same conceptual properties as a normal grace period, but the
+expedited implementation accepts extreme CPU overhead to obtain
+much lower grace-period latency, for example, consuming a few
+tens of microseconds worth of CPU time to reduce grace-period
+duration from milliseconds to tens of microseconds.
+The <tt>->qsmaskinit</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children cover for at least
+one online CPU.
+This mask is used to initialize <tt>->qsmask</tt>,
+and <tt>->expmaskinit</tt> is used to initialize
+<tt>->expmask</tt> and the beginning of the
+normal and expedited grace periods, respectively.
+
+<p><a name="Quick Quiz 4"><b>Quick Quiz 4</b>:</a>
+Why are these bitmasks protected by locking?
+Come on, haven't you heard of atomic instructions???
+<br><a href="#qq4answer">Answer</a>
+
+<h5>Blocked-Task Management</h5>
+
+<p><tt>TREE_PREEMPT_RCU</tt> allows tasks to be preempted in the
+midst of their RCU read-side critical sections, and these tasks
+must be tracked explicitly.
+The details of exactly why and how they are tracked will be covered
+in a separate article on RCU read-side processing.
+For now, it is enough to know that the <tt>rcu_node</tt>
+structure tracks them.
+
+<pre>
+ 1 struct list_head blkd_tasks;
+ 2 struct list_head *gp_tasks;
+ 3 struct list_head *exp_tasks;
+ 4 bool wait_blkd_tasks;
+</pre>
+
+<p>The <tt>->blkd_tasks</tt> field is a list header for
+the list of blocked and preempted tasks.
+As tasks undergo context switches within RCU read-side critical
+sections, their <tt>task_struct</tt> structures are enqueued
+(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt>
+field) onto the head of the <tt>->blkd_tasks</tt> list for the
+leaf <tt>rcu_node</tt> structure corresponding to the CPU
+on which the outgoing context switch executed.
+As these tasks later exit their RCU read-side critical sections,
+they remove themselves from the list.
+This list is therefore in reverse time order, so that if one of the tasks
+is blocking the current grace period, all subsequent tasks must
+also be blocking that same grace period.
+Therefore, a single pointer into this list suffices to track
+all tasks blocking a given grace period.
+That pointer is stored in <tt>->gp_tasks</tt> for normal
+grace periods and in <tt>->exp_tasks</tt> for expedited
+grace periods.
+These last two fields are <tt>NULL</tt> if either there is
+no grace period in flight or if there are no blocked tasks
+preventing that grace period from completing.
+If either of these two pointers is referencing a task that
+removes itself from the <tt>->blkd_tasks</tt> list,
+then that task must advance the pointer to the next task on
+the list, or set the pointer to <tt>NULL</tt> if there
+are no subsequent tasks on the list.
+
+</p><p>For example, suppose that tasks T1, T2, and T3 are
+all hard-affinitied to the largest-numbered CPU in the system.
+Then if task T1 blocked in an RCU read-side
+critical section, then an expedited grace period started,
+then task T2 blocked in an RCU read-side critical section,
+then a normal grace period started, and finally task 3 blocked
+in an RCU read-side critical section, then the state of the
+last leaf <tt>rcu_node</tt> structure's blocked-task list
+would be as shown below:
+
+</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
+
+</p><p>Task T1 is blocking both grace periods, task T2 is
+blocking only the normal grace period, and task T3 is blocking
+neither grace period.
+Note that these tasks will not remove themselves from this list
+immediately upon resuming execution.
+They will instead remain on the list until they execute the outermost
+<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
+section.
+
+<p>
+The <tt>->wait_blkd_tasks</tt> field indicates whether or not
+the current grace period is waiting on a blocked task.
+
+<h5>Sizing the <tt>rcu_node</tt> Array</h5>
+
+<p>The <tt>rcu_node</tt> array is sized via a series of
+C-preprocessor expressions as follows:
+
+<pre>
+ 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS <= RCU_FANOUT_1
+27 # define RCU_NUM_LVLS 1
+28 # define NUM_RCU_LVL_0 1
+29 # define NUM_RCU_NODES NUM_RCU_LVL_0
+30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
+31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
+32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
+33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
+34 #elif NR_CPUS <= RCU_FANOUT_2
+35 # define RCU_NUM_LVLS 2
+36 # define NUM_RCU_LVL_0 1
+37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
+41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS <= RCU_FANOUT_3
+44 # define RCU_NUM_LVLS 3
+45 # define NUM_RCU_LVL_0 1
+46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS <= RCU_FANOUT_4
+54 # define RCU_NUM_LVLS 4
+55 # define NUM_RCU_LVL_0 1
+56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+</pre>
+
+<p>The maximum number of levels in the <tt>rcu_node</tt> structure
+is currently limited to four, as specified by lines 21-24
+and the structure of the subsequent “if” statement.
+For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
+should be sufficient for the next few years at least.
+For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
+should see us through the next decade or so.
+This four-level tree also allows kernels built with
+<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
+which might be useful in very large systems having eight CPUs per
+socket (but please note that no one has yet shown any measurable
+performance degradation due to misaligned socket and <tt>rcu_node</tt>
+boundaries).
+In addition, building kernels with a full four levels of <tt>rcu_node</tt>
+tree permits better testing of RCU's combining-tree code.
+
+</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
+are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
+If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
+it is set based on the word size of the system, which is also
+the Kconfig default.
+
+</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
+handled by each leaf <tt>rcu_node</tt> structure.
+Experience has shown that allowing a given leaf <tt>rcu_node</tt>
+structure to handle 64 CPUs, as permitted by the number of bits in
+the <tt>->qsmask</tt> field on a 64-bit system, results in
+excessive contention for the leaf <tt>rcu_node</tt> structures'
+<tt>->lock</tt> fields.
+The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
+limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
+If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
+selected is based on the word size of the system, just as for
+<tt>CONFIG_RCU_FANOUT</tt>.
+Lines 11-19 perform this computation.
+
+</p><p>Lines 21-24 compute the maximum number of CPUs supported by
+a single-level (which contains a single <tt>rcu_node</tt> structure),
+two-level, three-level, and four-level <tt>rcu_node</tt> tree,
+respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
+and <tt>RCU_FANOUT_LEAF</tt>.
+These numbers of CPUs are retained in the
+<tt>RCU_FANOUT_1</tt>,
+<tt>RCU_FANOUT_2</tt>,
+<tt>RCU_FANOUT_3</tt>, and
+<tt>RCU_FANOUT_4</tt>
+C-preprocessor variables, respectively.
+
+</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
+statement spanning lines 26-66 that computes the number of
+<tt>rcu_node</tt> structures required for each level of the tree,
+as well as the number of levels required.
+The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
+C-preprocessor variable by lines 27, 35, 44, and 54.
+The number of <tt>rcu_node</tt> structures for the topmost level
+of the tree is always exactly one, and this value is unconditionally
+placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55.
+The rest of the levels (if any) of the <tt>rcu_node</tt> tree
+are computed by dividing the maximum number of CPUs by the
+fanout supported by the number of levels from the current level down,
+rounding up. This computation is performed by lines 37,
+46-47, and 56-58.
+Lines 31-33, 40-42, 50-52, and 62-63 create initializers
+for lockdep lock-class names.
+Finally, lines 64-66 produce an error if the maximum number of
+CPUs is too large for the specified fanout.
+
+<h3><a name="The rcu_data Structure">
+The <tt>rcu_data</tt> Structure</a></h3>
+
+<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
+corresponding flavor of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+This structure is the
+focus of quiescent-state detection and RCU callback queuing.
+It also tracks its relationship to the corresponding leaf
+<tt>rcu_node</tt> structure to allow more-efficient
+propagation of quiescent states up the <tt>rcu_node</tt>
+combining tree.
+Like the <tt>rcu_node</tt> structure, it provides a local
+copy of the grace-period information to allow for-free
+synchronized
+access to this information from the corresponding CPU.
+Finally, this structure records past dyntick-idle state
+for the corresponding CPU and also tracks statistics.
+
+</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Other Data Structures</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 int cpu;
+ 2 struct rcu_state *rsp;
+ 3 struct rcu_node *mynode;
+ 4 struct rcu_dynticks *dynticks;
+ 5 unsigned long grpmask;
+ 6 bool beenonline;
+</pre>
+
+<p>The <tt>->cpu</tt> field contains the number of the
+corresponding CPU, the <tt>->rsp</tt> pointer references
+the corresponding <tt>rcu_state</tt> structure (and is most frequently
+used to locate the name of the corresponding flavor of RCU for tracing),
+and the <tt>->mynode</tt> field references the corresponding
+<tt>rcu_node</tt> structure.
+The <tt>->mynode</tt> is used to propagate quiescent states
+up the combining tree.
+<p>The <tt>->dynticks</tt> pointer references the
+<tt>rcu_dynticks</tt> structure corresponding to this
+CPU.
+Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
+structure is shared among all flavors of RCU.
+These first four fields are constant and therefore require not
+synchronization.
+
+</p><p>The <tt>->grpmask</tt> field indicates the bit in
+the <tt>->mynode->qsmask</tt> corresponding to this
+<tt>rcu_data</tt> structure, and is also used when propagating
+quiescent states.
+The <tt>->beenonline</tt> flag is set whenever the corresponding
+CPU comes online, which means that the debugfs tracing need not dump
+out any <tt>rcu_data</tt> structure for which this flag is not set.
+
+<h5>Quiescent-State and Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long completed;
+ 2 unsigned long gpnum;
+ 3 bool cpu_no_qs;
+ 4 bool core_needs_qs;
+ 5 bool gpwrap;
+ 6 unsigned long rcu_qs_ctr_snap;
+</pre>
+
+<p>The <tt>completed</tt> and <tt>gpnum</tt>
+fields are the counterparts of the fields of the same name
+in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
+They may each lag up to one behind their <tt>rcu_node</tt>
+counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
+<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
+arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
+will catch up upon exit from dyntick-idle mode).
+If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and
+<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
+structure,
+the <tt>->gpnum</tt> field will be one greater than the
+<tt>->complete</tt> fields, with <tt>->gpnum</tt>
+indicating which grace period this <tt>rcu_data</tt> believes
+is still being waited for.
+
+<p><a name="Quick Quiz 5"><b>Quick Quiz 5</b>:</a>
+All this replication of the grace period numbers can only cause
+massive confusion.
+Why not just keep a global pair of counters and be done with it???
+<br><a href="#qq5answer">Answer</a>
+
+</p><p>The <tt>->cpu_no_qs</tt> flag indicates that the
+CPU has not yet passed through a quiescent state,
+while the <tt>->core_needs_qs</tt> flag indicates that the
+RCU core needs a quiescent state from the corresponding CPU.
+The <tt>->gpwrap</tt> field indicates that the corresponding
+CPU has remained idle for so long that the <tt>completed</tt>
+and <tt>gpnum</tt> counters are in danger of overflow, which
+will cause the CPU to disregard the values of its counters on
+its next exit from idle.
+Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
+cases where a given operation has resulted in a quiescent state
+for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>.
+
+<h5>RCU Callback Handling</h5>
+
+<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
+the same CPU that registered them.
+This is strictly a cache-locality optimization: callbacks can and
+do get invoked on CPUs other than the one that registered them.
+After all, if the CPU that registered a given callback has gone
+offline before the callback can be invoked, there really is no other
+choice.
+
+</p><p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_head *nxtlist;
+ 2 struct rcu_head **nxttail[RCU_NEXT_SIZE];
+ 3 unsigned long nxtcompleted[RCU_NEXT_SIZE];
+ 4 long qlen_lazy;
+ 5 long qlen;
+ 6 long qlen_last_fqs_check;
+ 7 unsigned long n_force_qs_snap;
+ 8 unsigned long n_cbs_invoked;
+ 9 unsigned long n_cbs_orphaned;
+10 unsigned long n_cbs_adopted;
+11 long blimit;
+</pre>
+
+<p>The <tt>->nxtlist</tt> pointer and the
+<tt>->nxttail[]</tt> array form a four-segment list with
+older callbacks near the head and newer ones near the tail.
+Each segment contains callbacks with the corresponding relationship
+to the current grace period.
+The pointer out of the end of each of the four segments is referenced
+by the element of the <tt>->nxttail[]</tt> array indexed by
+<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period),
+<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period),
+<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next
+grace period), and
+<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated
+with a specific grace period)
+respectively, as shown in the following figure.
+
+</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
+
+</p><p>In this figure, the <tt>->nxtlist</tt> pointer references the
+first
+RCU callback in the list.
+The <tt>->nxttail[RCU_DONE_TAIL]</tt> array element references
+the <tt>->nxtlist</tt> pointer itself, indicating that none
+of the callbacks is ready to invoke.
+The <tt>->nxttail[RCU_WAIT_TAIL]</tt> array element references callback
+CB 2's <tt>->next</tt> pointer, which indicates that
+CB 1 and CB 2 are both waiting on the current grace period.
+The <tt>->nxttail[RCU_NEXT_READY_TAIL]</tt> array element
+references the same RCU callback that <tt>->nxttail[RCU_WAIT_TAIL]</tt>
+does, which indicates that there are no callbacks waiting on the next
+RCU grace period.
+The <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element references
+CB 4's <tt>->next</tt> pointer, indicating that all the
+remaining RCU callbacks have not yet been assigned to an RCU grace
+period.
+Note that the <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element
+always references the last RCU callback's <tt>->next</tt> pointer
+unless the callback list is empty, in which case it references
+the <tt>->nxtlist</tt> pointer.
+
+</p><p>CPUs advance their callbacks from the
+<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
+<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
+as grace periods advance.
+The CPU advances the callbacks in its <tt>rcu_data</tt> structure
+whenever it notices that another RCU grace period has completed.
+The CPU detects the completion of an RCU grace period by noticing
+that the value of its <tt>rcu_data</tt> structure's
+<tt>->completed</tt> field differs from that of its leaf
+<tt>rcu_node</tt> structure.
+Recall that each <tt>rcu_node</tt> structure's
+<tt>->completed</tt> field is updated at the end of each
+grace period.
+
+</p><p>The <tt>->nxtcompleted[]</tt> array records grace-period
+numbers corresponding to the list segments.
+This allows CPUs that go idle for extended periods to determine
+which of their callbacks are ready to be invoked after reawakening.
+
+</p><p>The <tt>->qlen</tt> counter contains the number of
+callbacks in <tt>->nxtlist</tt>, and the
+<tt>->qlen_lazy</tt> contains the number of those callbacks that
+are known to only free memory, and whose invocation can therefore
+be safely deferred.
+The <tt>->qlen_last_fqs_check</tt> and
+<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent
+states from <tt>call_rcu()</tt> and friends when callback
+lists grow excessively long.
+
+</p><p>The <tt>->n_cbs_invoked</tt>,
+<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt>
+fields count the number of callbacks invoked,
+sent to other CPUs when this CPU goes offline,
+and received from other CPUs when those other CPUs go offline.
+Finally, the <tt>->blimit</tt> counter is the maximum number of
+RCU callbacks that may be invoked at a given time.
+
+<h5>Dyntick-Idle Handling</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 int dynticks_snap;
+ 2 unsigned long dynticks_fqs;
+</pre>
+
+The <tt>->dynticks_snap</tt> field is used to take a snapshot
+of the corresponding CPU's dyntick-idle state when forcing
+quiescent states, and is therefore accessed from other CPUs.
+Finally, the <tt>->dynticks_fqs</tt> field is used to
+count the number of times this CPU is determined to be in
+dyntick-idle state, and is used for tracing and debugging purposes.
+
+<h3><a name="The rcu_dynticks Structure">
+The <tt>rcu_dynticks</tt> Structure</a></h3>
+
+<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
+for the corresponding CPU.
+Unlike the other structures, <tt>rcu_dynticks</tt> is not
+replicated over the different flavors of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+Its fields are as follows:
+
+<pre>
+ 1 int dynticks_nesting;
+ 2 int dynticks_nmi_nesting;
+ 3 atomic_t dynticks;
+</pre>
+
+<p>The <tt>->dynticks_nesting</tt> field counts the
+nesting depth of normal interrupts.
+In addition, this counter is incremented when exiting dyntick-idle
+mode and decremented when entering it.
+This counter can therefore be thought of as counting the number
+of reasons why this CPU cannot be permitted to enter dyntick-idle
+mode, aside from non-maskable interrupts (NMIs).
+NMIs are counted by the <tt>->dynticks_nmi_nesting</tt>
+field, except that NMIs that interrupt non-dyntick-idle execution
+are not counted.
+
+</p><p>Finally, the <tt>->dynticks</tt> field counts the corresponding
+CPU's transitions to and from dyntick-idle mode, so that this counter
+has an even value when the CPU is in dyntick-idle mode and an odd
+value otherwise.
+
+<p><a name="Quick Quiz 6"><b>Quick Quiz 6</b>:</a>
+Why not just count all NMIs?
+Wouldn't that be simpler and less error prone?
+<br><a href="#qq6answer">Answer</a>
+
+</p><p>Additional fields are present for some special-purpose
+builds, and are discussed separately.
+
+<h3><a name="The rcu_head Structure">
+The <tt>rcu_head</tt> Structure</a></h3>
+
+<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
+These structures are normally embedded within RCU-protected data
+structures whose algorithms use asynchronous grace periods.
+In contrast, when using algorithms that block waiting for RCU grace periods,
+RCU users need not provide <tt>rcu_head</tt> structures.
+
+</p><p>The <tt>rcu_head</tt> structure has fields as follows:
+
+<pre>
+ 1 struct rcu_head *next;
+ 2 void (*func)(struct rcu_head *head);
+</pre>
+
+<p>The <tt>->next</tt> field is used
+to link the <tt>rcu_head</tt> structures together in the
+lists within the <tt>rcu_data</tt> structures.
+The <tt>->func</tt> field is a pointer to the function
+to be called when the callback is ready to be invoked, and
+this function is passed a pointer to the <tt>rcu_head</tt>
+structure.
+However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt>
+field to record the offset of the <tt>rcu_head</tt>
+structure within the enclosing RCU-protected data structure.
+
+</p><p>Both of these fields are used internally by RCU.
+From the viewpoint of RCU users, this structure is an
+opaque “cookie”.
+
+<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
+Given that the callback function <tt>->func</tt>
+is passed a pointer to the <tt>rcu_head</tt> structure,
+how is that function supposed to find the beginning of the
+enclosing RCU-protected data structure?
+<br><a href="#qq7answer">Answer</a>
+
+<h3><a name="RCU-Specific Fields in the task_struct Structure">
+RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
+
+<p>The <tt>CONFIG_TREE_PREEMPT_RCU</tt> implementation uses some
+additional fields in the <tt>task_struct</tt> structure:
+
+<pre>
+ 1 #ifdef CONFIG_PREEMPT_RCU
+ 2 int rcu_read_lock_nesting;
+ 3 union rcu_special rcu_read_unlock_special;
+ 4 struct list_head rcu_node_entry;
+ 5 struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8 unsigned long rcu_tasks_nvcsw;
+ 9 bool rcu_tasks_holdout;
+10 struct list_head rcu_tasks_holdout_list;
+11 int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+</pre>
+
+<p>The <tt>->rcu_read_lock_nesting</tt> field records the
+nesting level for RCU read-side critical sections, and
+the <tt>->rcu_read_unlock_special</tt> field is a bitmask
+that records special conditions that require <tt>rcu_read_unlock()</tt>
+to do additional work.
+The <tt>->rcu_node_entry</tt> field is used to form lists of
+tasks that have blocked within preemptible-RCU read-side critical
+sections and the <tt>->rcu_blocked_node</tt> field references
+the <tt>rcu_node</tt> structure whose list this task is a member of,
+or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
+read-side critical section.
+
+<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of
+voluntary context switches that this task had undergone at the
+beginning of the current tasks-RCU grace period,
+<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU
+grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt>
+is a list element enqueuing this task on the holdout list,
+and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this
+idle task is running, but only if the task is currently running,
+that is, if the CPU is currently idle.
+
+<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
+Why is <tt>->rcu_boosted</tt> required, given that there is
+a <tt>RCU_READ_UNLOCK_BOOSTED</tt> bit in
+<tt>->rcu_read_unlock_special</tt>?
+<br><a href="#qq8answer">Answer</a>
+
+<h3><a name="Accessor Functions">
+Accessor Functions</a></h3>
+
+<p>The following listing shows the
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_for_each_leaf_node()</tt> function and macros:
+
+<pre>
+ 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+ 2 {
+ 3 return &rsp->node[0];
+ 4 }
+ 5
+ 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+ 7 for ((rnp) = &(rsp)->node[0]; \
+ 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+ 9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11 for ((rnp) = &(rsp)->node[0]; \
+ 12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
+ 16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+</pre>
+
+<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
+first element of the specified <tt>rcu_state</tt> structure's
+<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt>
+structure.
+
+</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
+macro takes advantage of the layout of the <tt>rcu_node</tt>
+structures in the <tt>rcu_state</tt> structure's
+<tt>->node[]</tt> array, performing a breadth-first traversal by
+simply traversing the array in order.
+The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
+similarly, but traverses only the first part of the array, thus excluding
+the leaf <tt>rcu_node</tt> structures.
+Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+the last part of the array, thus traversing only the leaf
+<tt>rcu_node</tt> structures.
+
+<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
+What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+contains only a single node?
+<br><a href="#qq9answer">Answer</a>
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
+which contains a combining tree of <tt>rcu_node</tt> and
+<tt>rcu_data</tt> structures.
+Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
+state is tracked by an <tt>rcu_dynticks</tt> structure.
+
+If you made it this far, you are well prepared to read the code
+walkthroughs in the other articles in this series.
+
+<h3><a name="Acknowledgments">
+Acknowledgments</a></h3>
+
+I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
+Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
+for helping me get this document into a more human-readable state.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+
+<h3><a name="Answers to Quick Quizzes">
+Answers to Quick Quizzes</a></h3>
+
+<a name="qq1answer"></a>
+<p><b>Quick Quiz 1</b>:
+Why isn't the fanout at the leaves also 64?
+
+
+</p><p><b>Answer</b>:
+Because there are more types of events that affect the leaf-level
+<tt>rcu_node</tt> structures than further up the tree.
+Therefore, if the leaf <tt>rcu_node</tt> structures have
+fanout of 64, the contention on these structures' <tt>->structures</tt>
+becomes excessive.
+Experimentation on a wide variety of systems has shown that a fanout
+of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
+
+</p><p>Of course, further experience with systems having hundreds or
+thousands of CPUs may demonstrate that the fanout for the non-leaf
+<tt>rcu_node</tt> structures must also be reduced.
+Such reduction can be easily carried out when and if it proves necessary.
+In the meantime, if you are using such a system and running into
+contention problems on the non-leaf <tt>rcu_node</tt> structures,
+you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
+parameter to reduce the non-leaf fanout as needed.
+
+</p><p>Kernels built for systems with strong NUMA characteristics might
+also need to adjust <tt>CONFIG_RCU_FANOUT</tt> so that the
+domains of the <tt>rcu_node</tt> structures align with hardware
+boundaries.
+However, there has thus far been no need for this.
+
+
+</p><p><a href="#Quick%20Quiz%201"><b>Back to Quick Quiz 1</b>.</a>
+
+<a name="qq2answer"></a>
+<p><b>Quick Quiz 2</b>:
+Wait a minute!
+You said that the <tt>rcu_node</tt> structures formed a tree,
+but they are declared as a flat array!
+What gives?
+
+
+</p><p><b>Answer</b>:
+The tree is laid out in the array.
+The first node In the array is the head, the next set of nodes in the
+array are children of the head node, and so on until the last set of
+nodes in the array are the leaves.
+
+</p><p>See the following diagrams to see how this works.
+
+
+</p><p><a href="#Quick%20Quiz%202"><b>Back to Quick Quiz 2</b>.</a>
+
+<a name="qq3answer"></a>
+<p><b>Quick Quiz 3</b>:
+Given that this array represents a tree, why can't the diagram that
+includes the <tt>->level</tt> array be planar?
+
+
+</p><p><b>Answer</b>:
+It can be planar, it is just that it looks uglier that way.
+But don't take my word for it, draw it yourself!
+
+</p><p>But if you draw the tree to be tree-shaped rather than
+array-shaped, it is easy to draw a planar representation:
+
+</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
+
+
+</p><p><a href="#Quick%20Quiz%203"><b>Back to Quick Quiz 3</b>.</a>
+
+<a name="qq4answer"></a>
+<p><b>Quick Quiz 4</b>:
+Why are these bitmasks protected by locking?
+Come on, haven't you heard of atomic instructions???
+
+
+</p><p><b>Answer</b>:
+Lockless grace-period computation! Such a tantalizing possibility!
+
+</p><p>But consider the following sequence of events:
+
+<ol>
+<li> CPU 0 has been in dyntick-idle mode for quite
+ some time.
+ When it wakes up, it notices that the current RCU
+ grace period needs it to report in, so it sets a
+ flag where the scheduling clock interrupt will find it.
+<li> Meanwhile, CPU 1 is running <tt>force_quiescent_state()</tt>,
+ and notices that CPU 0 has been in dyntick idle mode,
+ which qualifies as an extended quiescent state.
+<li> CPU 0's scheduling clock interrupt fires in the
+ middle of an RCU read-side critical section, and notices
+ that the RCU core needs something, so commences RCU softirq
+ processing.
+<li> CPU 0's softirq handler executes and is just about ready
+ to report its quiescent state up the <tt>rcu_node</tt>
+ tree.
+<li> But CPU 1 beats it to the punch, completing the current
+ grace period and starting a new one.
+<li> CPU 0 now reports its quiescent state for the wrong
+ grace period.
+ That grace period might now end before the RCU read-side
+ critical section.
+ If that happens, disaster will ensue.
+</ol>
+
+<p>So the locking is absolutely required in order to coordinate clearing
+of the bits with the grace-period numbers in <tt>->gpnum</tt>
+and <tt>->completed</tt>.
+
+
+</p><p><a href="#Quick%20Quiz%204"><b>Back to Quick Quiz 4</b>.</a>
+
+<a name="qq5answer"></a>
+<p><b>Quick Quiz 5</b>:
+All this replication of the grace period numbers can only cause
+massive confusion.
+Why not just keep a global pair of counters and be done with it???
+
+
+</p><p><b>Answer</b>:
+Because if there was only a single global pair of grace-period numbers,
+there would need to be a single global lock to allow safely accessing
+and updating them.
+And if we are not going to have a single global lock, we need to carefully
+manage the numbers on a per-node basis.
+Recall from the answer to a previous Quick Quiz that the consequences
+of applying a previously sampled quiescent state to the wrong
+grace period are quite severe.
+
+
+</p><p><a href="#Quick%20Quiz%205"><b>Back to Quick Quiz 5</b>.</a>
+
+<a name="qq6answer"></a>
+<p><b>Quick Quiz 6</b>:
+Why not just count all NMIs?
+Wouldn't that be simpler and less error prone?
+
+
+</p><p><b>Answer</b>:
+It seems simpler only until you think hard about how to go about
+updating the <tt>rcu_dynticks</tt> structure's
+<tt>->dynticks</tt> field.
+
+
+</p><p><a href="#Quick%20Quiz%206"><b>Back to Quick Quiz 6</b>.</a>
+
+<a name="qq7answer"></a>
+<p><b>Quick Quiz 7</b>:
+Given that the callback function <tt>->func</tt>
+is passed a pointer to the <tt>rcu_head</tt> structure,
+how is that function supposed to find the beginning of the
+enclosing RCU-protected data structure?
+
+
+</p><p><b>Answer</b>:
+In actual practice, there is a separate callback function per
+type of RCU-protected data structure.
+The callback function can therefore use the <tt>container_of()</tt>
+macro in the Linux kernel (or other pointer-manipulation facilities
+in other software environments) to find the beginning of the
+enclosing structure.
+
+
+</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a>
+
+<a name="qq8answer"></a>
+<p><b>Quick Quiz 8</b>:
+Why is <tt>->rcu_boosted</tt> required, given that there is
+a <tt>RCU_READ_UNLOCK_BOOSTED</tt> bit in
+<tt>->rcu_read_unlock_special</tt>?
+
+
+</p><p><b>Answer</b>:
+The <tt>->rcu_read_unlock_special</tt> field may only be
+updated by the task itself.
+By definition, RCU priority boosting must be carried out by some
+other task.
+This other task cannot safely update the boosted task's
+<tt>->rcu_read_unlock_special</tt> field without the use of
+expensive atomic instructions.
+The <tt>->rcu_boosted</tt> field is therefore used by the
+boosting task to let the boosted task know that it has been boosted.
+The boosted task makes use of the
+<tt>RCU_READ_UNLOCK_BOOSTED</tt> bit in
+<tt>->rcu_read_unlock_special</tt>
+when deboosting itself.
+
+
+</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
+
+<a name="qq9answer"></a>
+<p><b>Quick Quiz 9</b>:
+What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+contains only a single node?
+
+
+</p><p><b>Answer</b>:
+In the single-node case,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
+and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+
+
+</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
+
+
+
+</body></html>
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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
+ "http://www.w3.org/TR/html4/loose.dtd">
+ <html>
+ <head><title>A Tour Through TREE_RCU's Data Structures [LWN.net]</title>
+ <meta HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
+
+ <p>January 27, 2016</p>
+ <p>This article was contributed by Paul E. McKenney</p>
+
+<h3>Introduction</h3>
+
+This document describes RCU's major data structures and their relationship
+to each other.
+
+<ol>
+<li> <a href="#Data-Structure Relationships">
+ Data-Structure Relationships</a>
+<li> <a href="#The rcu_state Structure">
+ The <tt>rcu_state</tt> Structure</a>
+<li> <a href="#The rcu_node Structure">
+ The <tt>rcu_node</tt> Structure</a>
+<li> <a href="#The rcu_data Structure">
+ The <tt>rcu_data</tt> Structure</a>
+<li> <a href="#The rcu_dynticks Structure">
+ The <tt>rcu_dynticks</tt> Structure</a>
+<li> <a href="#The rcu_head Structure">
+ The <tt>rcu_head</tt> Structure</a>
+<li> <a href="#RCU-Specific Fields in the task_struct Structure">
+ RCU-Specific Fields in the <tt>task_struct</tt> Structure</a>
+<li> <a href="#Accessor Functions">
+ Accessor Functions</a>
+</ol>
+
+At the end we have the
+<a href="#Answers to Quick Quizzes">answers to the quick quizzes</a>.
+
+<h3><a name="Data-Structure Relationships">Data-Structure Relationships</a></h3>
+
+<p>RCU is for all intents and purposes a large state machine, and its
+data structures maintain the state in such a way as to allow RCU readers
+to execute extremely quickly, while also processing the RCU grace periods
+requested by updaters in an efficient and extremely scalable fashion.
+The efficiency and scalability of RCU updaters is provided primarily
+by a combining tree, as shown below:
+
+</p><p><img src="BigTreeClassicRCU.svg" alt="BigTreeClassicRCU.svg" width="30%">
+
+</p><p>This diagram shows an enclosing <tt>rcu_state</tt> structure
+containing a tree of <tt>rcu_node</tt> structures.
+Each leaf node of the <tt>rcu_node</tt> tree has up to 16
+<tt>rcu_data</tt> structures associated with it, so that there
+are <tt>NR_CPUS</tt> number of <tt>rcu_data</tt> structures,
+one for each possible CPU.
+This structure is adjusted at boot time, if needed, to handle the
+common case where <tt>nr_cpu_ids</tt> is much less than
+<tt>NR_CPUs</tt>.
+For example, a number of Linux distributions set <tt>NR_CPUs=4096</tt>,
+which results in a three-level <tt>rcu_node</tt> tree.
+If the actual hardware has only 16 CPUs, RCU will adjust itself
+at boot time, resulting in an <tt>rcu_node</tt> tree with only a single node.
+
+</p><p>The purpose of this combining tree is to allow per-CPU events
+such as quiescent states, dyntick-idle transitions,
+and CPU hotplug operations to be processed efficiently
+and scalably.
+Quiescent states are recorded by the per-CPU <tt>rcu_data</tt> structures,
+and other events are recorded by the leaf-level <tt>rcu_node</tt>
+structures.
+All of these events are combined at each level of the tree until finally
+grace periods are completed at the tree's root <tt>rcu_node</tt>
+structure.
+A grace period can be completed at the root once every CPU
+(or, in the case of <tt>CONFIG_TREE_PREEMPT_RCU</tt>, task)
+has passed through a quiescent state.
+Once a grace period has completed, record of that fact is propagated
+back down the tree.
+
+</p><p>As can be seen from the diagram, on a 64-bit system
+a two-level tree with 64 leaves can accommodate 1,024 CPUs, with a fanout
+of 64 at the root and a fanout of 16 at the leaves.
+
+<p>@@QQ@@
+Why isn't the fanout at the leaves also 64?
+<p>@@QQA@@
+Because there are more types of events that affect the leaf-level
+<tt>rcu_node</tt> structures than further up the tree.
+Therefore, if the leaf <tt>rcu_node</tt> structures have
+fanout of 64, the contention on these structures' <tt>->structures</tt>
+becomes excessive.
+Experimentation on a wide variety of systems has shown that a fanout
+of 16 works well for the leaves of the <tt>rcu_node</tt> tree.
+
+</p><p>Of course, further experience with systems having hundreds or
+thousands of CPUs may demonstrate that the fanout for the non-leaf
+<tt>rcu_node</tt> structures must also be reduced.
+Such reduction can be easily carried out when and if it proves necessary.
+In the meantime, if you are using such a system and running into
+contention problems on the non-leaf <tt>rcu_node</tt> structures,
+you may use the <tt>CONFIG_RCU_FANOUT</tt> kernel configuration
+parameter to reduce the non-leaf fanout as needed.
+
+</p><p>Kernels built for systems with strong NUMA characteristics might
+also need to adjust <tt>CONFIG_RCU_FANOUT</tt> so that the
+domains of the <tt>rcu_node</tt> structures align with hardware
+boundaries.
+However, there has thus far been no need for this.
+<p>@@QQE@@
+
+</p><p>If your system has more than 1,024 CPUs (or more than 512 CPUs on
+a 32-bit system), then RCU will automatically add more levels to the
+tree.
+For example, if you are crazy enough to build a 64-bit system with 65,536
+CPUs, RCU would configure the <tt>rcu_node</tt> tree as follows:
+
+</p><p><img src="HugeTreeClassicRCU.svg" alt="HugeTreeClassicRCU.svg" width="50%">
+
+</p><p>RCU currently permits up to a four-level tree, which on a 64-bit system
+accommodates up to 4,194,304 CPUs, though only a mere 524,288 CPUs for
+32-bit systems.
+On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
+as small as 2 if you wish, which would permit only 16 CPUs, which
+is useful for testing.
+
+</p><p>The Linux kernel actually supports multiple flavors of RCU
+running concurrently, so RCU builds separate data structures for each
+flavor.
+For example, for <tt>CONFIG_TREE_RCU=y</tt> kernels, RCU provides
+rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBH.svg" alt="BigTreeClassicRCUBH.svg" width="33%">
+
+</p><p>Energy efficiency is increasingly important, and for that
+reason the Linux kernel provides <tt>CONFIG_NO_HZ_IDLE</tt>, which
+turns off the scheduling-clock interrupts on idle CPUs, which in
+turn allows those CPUs to attain deeper sleep states and to consume
+less energy.
+CPUs whose scheduling-clock interrupts have been turned off are
+said to be in <i>dyntick-idle mode</i>.
+RCU must handle dyntick-idle CPUs specially
+because RCU would otherwise wake up each CPU on every grace period,
+which would defeat the whole purpose of <tt>CONFIG_NO_HZ_IDLE</tt>.
+RCU uses the <tt>rcu_dynticks</tt> structure to track
+which CPUs are in dyntick idle mode, as shown below:
+
+</p><p><img src="BigTreeClassicRCUBHdyntick.svg" alt="BigTreeClassicRCUBHdyntick.svg" width="33%">
+
+</p><p>However, if a CPU is in dyntick-idle mode, it is in that mode
+for all flavors of RCU.
+Therefore, a single <tt>rcu_dynticks</tt> structure is allocated per
+CPU, and all of a given CPU's <tt>rcu_data</tt> structures share
+that <tt>rcu_dynticks</tt>, as shown in the figure.
+
+</p><p>Kernels built with <tt>CONFIG_TREE_PREEMPT_RCU</tt> support
+rcu_preempt in addition to rcu_sched and rcu_bh, as shown below:
+
+</p><p><img src="BigTreePreemptRCUBHdyntick.svg" alt="BigTreePreemptRCUBHdyntick.svg" width="35%">
+
+</p><p>RCU updaters wait for normal grace periods by registering
+RCU callbacks, either directly via <tt>call_rcu()</tt> and
+friends (namely <tt>call_rcu_bh()</tt> and <tt>call_rcu_sched()</tt>),
+there being a separate interface per flavor of RCU)
+or indirectly via <tt>synchronize_rcu()</tt> and friends.
+RCU callbacks are represented by <tt>rcu_head</tt> structures,
+which are queued on <tt>rcu_data</tt> structures while they are
+waiting for a grace period to elapse, as shown in the following figure:
+
+</p><p><img src="BigTreePreemptRCUBHdyntickCB.svg" alt="BigTreePreemptRCUBHdyntickCB.svg" width="40%">
+
+</p><p>This figure shows how <tt>TREE_RCU</tt>'s and
+<tt>TREE_PREEMPT_RCU</tt>'s major data structures are related.
+Lesser data structures will be introduced with the algorithms that
+make use of them.
+
+</p><p>Note that each of the data structures in the above figure has
+its own synchronization:
+
+<p><ol>
+<li> Each <tt>rcu_state</tt> structures has a lock and a mutex,
+ and some fields are protected by the corresponding root
+ <tt>rcu_node</tt> structure's lock.
+<li> Each <tt>rcu_node</tt> structure has a spinlock.
+<li> The fields in <tt>rcu_data</tt> are private to the corresponding
+ CPU, although a few can be read and written by other CPUs.
+<li> Similarly, the fields in <tt>rcu_dynticks</tt> are private
+ to the corresponding CPU, although a few can be read by
+ other CPUs.
+</ol>
+
+<p>It is important to note that different data structures can have
+very different ideas about the state of RCU at any given time.
+For but one example, awareness of the start or end of a given RCU
+grace period propagates slowly through the data structures.
+This slow propagation is absolutely necessary for RCU to have good
+read-side performance.
+If this balkanized implementation seems foreign to you, one useful
+trick is to consider each instance of these data structures to be
+a different person, each having the usual slightly different
+view of reality.
+
+</p><p>The general role of each of these data structures is as
+follows:
+
+</p><ol>
+<li> <tt>rcu_state</tt>:
+ This structure forms the interconnection between the
+ <tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+ tracks grace periods, serves as short-term repository
+ for callbacks orphaned by CPU-hotplug events,
+ maintains <tt>rcu_barrier()</tt> state,
+ tracks expedited grace-period state,
+ and maintains state used to force quiescent states when
+ grace periods extend too long,
+<li> <tt>rcu_node</tt>: This structure forms the combining
+ tree that propagates quiescent-state
+ information from the leaves to the root, and also propagates
+ grace-period information from the root to the leaves.
+ It provides local copies of the grace-period state in order
+ to allow this information to be accessed in a synchronized
+ manner without suffering the scalability limitations that
+ would otherwise be imposed by global locking.
+ In <tt>CONFIG_TREE_PREEMPT_RCU</tt> kernels, it manages the lists
+ of tasks that have blocked while in their current
+ RCU read-side critical section.
+ In <tt>CONFIG_TREE_PREEMPT_RCU</tt> with
+ <tt>CONFIG_RCU_BOOST</tt>, it manages the
+ per-<tt>rcu_node</tt> priority-boosting
+ kernel threads (kthreads) and state.
+ Finally, it records CPU-hotplug state in order to determine
+ which CPUs should be ignored during a given grace period.
+<li> <tt>rcu_data</tt>: This per-CPU structure is the
+ focus of quiescent-state detection and RCU callback queuing.
+ It also tracks its relationship to the corresponding leaf
+ <tt>rcu_node</tt> structure to allow more-efficient
+ propagation of quiescent states up the <tt>rcu_node</tt>
+ combining tree.
+ Like the <tt>rcu_node</tt> structure, it provides a local
+ copy of the grace-period information to allow for-free
+ synchronized
+ access to this information from the corresponding CPU.
+ Finally, this structure records past dyntick-idle state
+ for the corresponding CPU and also tracks statistics.
+<li> <tt>rcu_dynticks</tt>:
+ This per-CPU structure tracks the current dyntick-idle
+ state for the corresponding CPU.
+ Unlike the other three structures, the <tt>rcu_dynticks</tt>
+ structure is not replicated per RCU flavor.
+<li> <tt>rcu_head</tt>:
+ This structure represents RCU callbacks, and is the
+ only structure allocated and managed by RCU users.
+ The <tt>rcu_head</tt> structure is normally embedded
+ within the RCU-protected data structure.
+</ol>
+
+<p>If all you wanted from this article was a general notion of how
+RCU's data structures are related, you are done.
+Otherwise, each of the following sections give more details on
+the <tt>rcu_state</tt>, <tt>rcu_node</tt>, <tt>rcu_data</tt>,
+and <tt>rcu_dynticks</tt> data structures.
+
+<h3><a name="The rcu_state Structure">
+The <tt>rcu_state</tt> Structure</a></h3>
+
+<p>The <tt>rcu_state</tt> structure is the base structure that
+represents a flavor of RCU.
+This structure forms the interconnection between the
+<tt>rcu_node</tt> and <tt>rcu_data</tt> structures,
+tracks grace periods, contains the lock used to
+synchronize with CPU-hotplug events,
+and maintains state used to force quiescent states when
+grace periods extend too long,
+
+</p><p>A few of the <tt>rcu_state</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+The more specialized fields are covered in the discussion of their
+use.
+
+<h5>Relationship to rcu_node and rcu_data Structures</h5>
+
+This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_node node[NUM_RCU_NODES];
+ 2 struct rcu_node *level[NUM_RCU_LVLS + 1];
+ 3 struct rcu_data __percpu *rda;
+</pre>
+
+<p>@@QQ@@
+Wait a minute!
+You said that the <tt>rcu_node</tt> structures formed a tree,
+but they are declared as a flat array!
+What gives?
+<p>@@QQA@@
+The tree is laid out in the array.
+The first node In the array is the head, the next set of nodes in the
+array are children of the head node, and so on until the last set of
+nodes in the array are the leaves.
+
+</p><p>See the following diagrams to see how this works.
+<p>@@QQE@@
+
+</p><p>The <tt>rcu_node</tt> tree is embedded into the
+<tt>->node[]</tt> array as shown in the following figure:
+
+</p><p><img src="TreeMapping.svg" alt="TreeMapping.svg" width="40%">
+
+</p><p>One interesting consequence of this mapping is that a
+breadth-first traversal of the tree is implemented as a simple
+linear scan of the array, which is in fact what the
+<tt>rcu_for_each_node_breadth_first()</tt> macro does.
+This macro is used at the beginning and ends of grace periods.
+
+</p><p>Each entry of the <tt>->level</tt> array references
+the first <tt>rcu_node</tt> structure on the corresponding level
+of the tree, for example, as shown below:
+
+</p><p><img src="TreeMappingLevel.svg" alt="TreeMappingLevel.svg" width="40%">
+
+</p><p>The zero<sup>th</sup> element of the array references the root
+<tt>rcu_node</tt> structure, the first element references the
+first child of the root <tt>rcu_node</tt>, and finally the second
+element references the first leaf <tt>rcu_node</tt> structure.
+
+<p>@@QQ@@
+Given that this array represents a tree, why can't the diagram that
+includes the <tt>->level</tt> array be planar?
+<p>@@QQA@@
+It can be planar, it is just that it looks uglier that way.
+But don't take my word for it, draw it yourself!
+
+</p><p>But if you draw the tree to be tree-shaped rather than
+array-shaped, it is easy to draw a planar representation:
+
+</p><p><img src="TreeLevel.svg" alt="TreeLevel.svg" width="60%">
+<p>@@QQE@@
+
+</p><p>Finally, the <tt>->rda</tt> field references a per-CPU
+pointer to the corresponding CPU's <tt>rcu_data</tt> structure.
+
+</p><p>All of these fields are constant once initialization is complete,
+and therefore need no protection.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gpnum;
+ 2 unsigned long completed;
+</pre>
+
+<p>RCU grace periods are numbered, and
+the <tt>->gpnum</tt> field contains the number of the grace
+period that started most recently.
+The <tt>->completed</tt> field contains the number of the
+grace period that completed most recently.
+If the two fields are equal, the RCU grace period that most recently
+started has already completed, and therefore the corresponding
+flavor of RCU is idle.
+If <tt>->gpnum</tt> is one greater than <tt>->completed</tt>,
+then <tt>->gpnum</tt> gives the number of the current RCU
+grace period, which has not yet completed.
+Any other combination of values indicates that something is broken.
+These two fields are protected by the root <tt>rcu_node</tt>'s
+<tt>->lock</tt> field.
+
+</p><p>There are <tt>->gpnum</tt> and <tt>->completed</tt> fields
+in the <tt>rcu_node</tt> and <tt>rcu_data</tt> structures
+as well.
+The fields in the <tt>rcu_state</tt> structure represent the
+most current values, and those of the other structures are compared
+in order to detect the start of a new grace period in a distributed
+fashion.
+The values flow from <tt>rcu_state</tt> to <tt>rcu_node</tt>
+(down the tree from the root to the leaves) to <tt>rcu_data</tt>.
+
+<h5>Miscellaneous</h5>
+
+<p>This portion of the <tt>rcu_state</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gp_max;
+ 2 char abbr;
+ 3 char *name;
+</pre>
+
+<p>The <tt>->gp_max</tt> field tracks the duration of the longest
+grace period in jiffies.
+It is protected by the root <tt>rcu_node</tt>'s <tt>->lock</tt>.
+
+<p>The <tt>->name</tt> field points to the name of the RCU flavor
+(for example, “rcu_sched”), and is constant.
+The <tt>->abbr</tt> field contains a one-character abbreviation,
+for example, “s” for RCU-sched.
+
+<h3><a name="The rcu_node Structure">
+The <tt>rcu_node</tt> Structure</a></h3>
+
+<p>The <tt>rcu_node</tt> structures form the combining
+tree that propagates quiescent-state
+information from the leaves to the root and also that propagates
+grace-period information from the root down to the leaves.
+They provides local copies of the grace-period state in order
+to allow this information to be accessed in a synchronized
+manner without suffering the scalability limitations that
+would otherwise be imposed by global locking.
+In <tt>CONFIG_TREE_PREEMPT_RCU</tt> kernels, they manage the lists
+of tasks that have blocked while in their current
+RCU read-side critical section.
+In <tt>CONFIG_TREE_PREEMPT_RCU</tt> with
+<tt>CONFIG_RCU_BOOST</tt>, they manage the
+per-<tt>rcu_node</tt> priority-boosting
+kernel threads (kthreads) and state.
+Finally, they record CPU-hotplug state in order to determine
+which CPUs should be ignored during a given grace period.
+
+</p><p>The <tt>rcu_node</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Combining Tree</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_node *parent;
+ 2 u8 level;
+ 3 u8 grpnum;
+ 4 unsigned long grpmask;
+ 5 int grplo;
+ 6 int grphi;
+</pre>
+
+<p>The <tt>->parent</tt> pointer references the <tt>rcu_node</tt>
+one level up in the tree, and is <tt>NULL</tt> for the root
+<tt>rcu_node</tt>.
+The RCU implementation makes heavy use of this field to push quiescent
+states up the tree.
+The <tt>->level</tt> field gives the level in the tree, with
+the root being at level zero, its children at level one, and so on.
+The <tt>->grpnum</tt> field gives this node's position within
+the children of its parent, so this number can range between 0 and 31
+on 32-bit systems and between 0 and 63 on 64-bit systems.
+The <tt>->level</tt> and <tt>->grpnum</tt> fields are
+used only during initialization and for tracing.
+The <tt>->grpmask</tt> field is the bitmask counterpart of
+<tt>->grpnum</tt>, and therefore always has exactly one bit set.
+This mask is used to clear the bit corresponding to this <tt>rcu_node</tt>
+structure in its parent's bitmasks, which are described later.
+Finally, the <tt>->grplo</tt> and <tt>->grphi</tt> fields
+contain the lowest and highest numbered CPU served by this
+<tt>rcu_node</tt> structure, respectively.
+
+</p><p>All of these fields are constant, and thus do not require any
+synchronization.
+
+<h5>Synchronization</h5>
+
+<p>This field of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 raw_spinlock_t lock;
+</pre>
+
+<p>This field is used to protect the remaining fields in this structure,
+unless otherwise stated.
+That said, all of the fields in this structure can be accessed without
+locking for tracing purposes.
+Yes, this can result in confusing traces, but better some tracing confusion
+than to be heisenbugged out of existence.
+
+<h5>Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_node</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long gpnum;
+ 2 unsigned long completed;
+</pre>
+
+<p>These fields are the counterparts of the fields of the same name in
+the <tt>rcu_state</tt> structure.
+They each may lag up to one behind their <tt>rcu_state</tt>
+counterparts.
+If a given <tt>rcu_node</tt> structure's <tt>->gpnum</tt> and
+<tt>->complete</tt> fields are equal, then this <tt>rcu_node</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> structure,
+the <tt>->gpnum</tt> field will be one greater than the
+<tt>->complete</tt> fields, with <tt>->gpnum</tt>
+indicating which grace period this <tt>rcu_node</tt> believes
+is still being waited for.
+
+</p><p>The <tt>>gpnum</tt> field of each <tt>rcu_node</tt>
+structure is updated at the beginning
+of each grace period, and the <tt>->completed</tt> fields are
+updated at the end of each grace period.
+
+<h5>Quiescent-State Tracking</h5>
+
+<p>These fields manage the propagation of quiescent states up the
+combining tree.
+
+</p><p>This portion of the <tt>rcu_node</tt> structure has fields
+as follows:
+
+<pre>
+ 1 unsigned long qsmask;
+ 2 unsigned long expmask;
+ 3 unsigned long qsmaskinit;
+ 4 unsigned long expmaskinit;
+</pre>
+
+<p>The <tt>->qsmask</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current normal grace period.
+Such children will have a value of 1 in their corresponding bit.
+Note that the leaf <tt>rcu_node</tt> structures should be
+thought of as having <tt>rcu_data</tt> structures as their
+children.
+Similarly, the <tt>->expmask</tt> field tracks which
+of this <tt>rcu_node</tt> structure's children still need to report
+quiescent states for the current expedited grace period.
+An expedited grace period has
+the same conceptual properties as a normal grace period, but the
+expedited implementation accepts extreme CPU overhead to obtain
+much lower grace-period latency, for example, consuming a few
+tens of microseconds worth of CPU time to reduce grace-period
+duration from milliseconds to tens of microseconds.
+The <tt>->qsmaskinit</tt> field tracks which of this
+<tt>rcu_node</tt> structure's children cover for at least
+one online CPU.
+This mask is used to initialize <tt>->qsmask</tt>,
+and <tt>->expmaskinit</tt> is used to initialize
+<tt>->expmask</tt> and the beginning of the
+normal and expedited grace periods, respectively.
+
+<p>@@QQ@@
+Why are these bitmasks protected by locking?
+Come on, haven't you heard of atomic instructions???
+<p>@@QQA@@
+Lockless grace-period computation! Such a tantalizing possibility!
+
+</p><p>But consider the following sequence of events:
+
+<ol>
+<li> CPU 0 has been in dyntick-idle mode for quite
+ some time.
+ When it wakes up, it notices that the current RCU
+ grace period needs it to report in, so it sets a
+ flag where the scheduling clock interrupt will find it.
+<li> Meanwhile, CPU 1 is running <tt>force_quiescent_state()</tt>,
+ and notices that CPU 0 has been in dyntick idle mode,
+ which qualifies as an extended quiescent state.
+<li> CPU 0's scheduling clock interrupt fires in the
+ middle of an RCU read-side critical section, and notices
+ that the RCU core needs something, so commences RCU softirq
+ processing.
+<li> CPU 0's softirq handler executes and is just about ready
+ to report its quiescent state up the <tt>rcu_node</tt>
+ tree.
+<li> But CPU 1 beats it to the punch, completing the current
+ grace period and starting a new one.
+<li> CPU 0 now reports its quiescent state for the wrong
+ grace period.
+ That grace period might now end before the RCU read-side
+ critical section.
+ If that happens, disaster will ensue.
+</ol>
+
+<p>So the locking is absolutely required in order to coordinate clearing
+of the bits with the grace-period numbers in <tt>->gpnum</tt>
+and <tt>->completed</tt>.
+<p>@@QQE@@
+
+<h5>Blocked-Task Management</h5>
+
+<p><tt>TREE_PREEMPT_RCU</tt> allows tasks to be preempted in the
+midst of their RCU read-side critical sections, and these tasks
+must be tracked explicitly.
+The details of exactly why and how they are tracked will be covered
+in a separate article on RCU read-side processing.
+For now, it is enough to know that the <tt>rcu_node</tt>
+structure tracks them.
+
+<pre>
+ 1 struct list_head blkd_tasks;
+ 2 struct list_head *gp_tasks;
+ 3 struct list_head *exp_tasks;
+ 4 bool wait_blkd_tasks;
+</pre>
+
+<p>The <tt>->blkd_tasks</tt> field is a list header for
+the list of blocked and preempted tasks.
+As tasks undergo context switches within RCU read-side critical
+sections, their <tt>task_struct</tt> structures are enqueued
+(via the <tt>task_struct</tt>'s <tt>->rcu_node_entry</tt>
+field) onto the head of the <tt>->blkd_tasks</tt> list for the
+leaf <tt>rcu_node</tt> structure corresponding to the CPU
+on which the outgoing context switch executed.
+As these tasks later exit their RCU read-side critical sections,
+they remove themselves from the list.
+This list is therefore in reverse time order, so that if one of the tasks
+is blocking the current grace period, all subsequent tasks must
+also be blocking that same grace period.
+Therefore, a single pointer into this list suffices to track
+all tasks blocking a given grace period.
+That pointer is stored in <tt>->gp_tasks</tt> for normal
+grace periods and in <tt>->exp_tasks</tt> for expedited
+grace periods.
+These last two fields are <tt>NULL</tt> if either there is
+no grace period in flight or if there are no blocked tasks
+preventing that grace period from completing.
+If either of these two pointers is referencing a task that
+removes itself from the <tt>->blkd_tasks</tt> list,
+then that task must advance the pointer to the next task on
+the list, or set the pointer to <tt>NULL</tt> if there
+are no subsequent tasks on the list.
+
+</p><p>For example, suppose that tasks T1, T2, and T3 are
+all hard-affinitied to the largest-numbered CPU in the system.
+Then if task T1 blocked in an RCU read-side
+critical section, then an expedited grace period started,
+then task T2 blocked in an RCU read-side critical section,
+then a normal grace period started, and finally task 3 blocked
+in an RCU read-side critical section, then the state of the
+last leaf <tt>rcu_node</tt> structure's blocked-task list
+would be as shown below:
+
+</p><p><img src="blkd_task.svg" alt="blkd_task.svg" width="60%">
+
+</p><p>Task T1 is blocking both grace periods, task T2 is
+blocking only the normal grace period, and task T3 is blocking
+neither grace period.
+Note that these tasks will not remove themselves from this list
+immediately upon resuming execution.
+They will instead remain on the list until they execute the outermost
+<tt>rcu_read_unlock()</tt> that ends their RCU read-side critical
+section.
+
+<p>
+The <tt>->wait_blkd_tasks</tt> field indicates whether or not
+the current grace period is waiting on a blocked task.
+
+<h5>Sizing the <tt>rcu_node</tt> Array</h5>
+
+<p>The <tt>rcu_node</tt> array is sized via a series of
+C-preprocessor expressions as follows:
+
+<pre>
+ 1 #ifdef CONFIG_RCU_FANOUT
+ 2 #define RCU_FANOUT CONFIG_RCU_FANOUT
+ 3 #else
+ 4 # ifdef CONFIG_64BIT
+ 5 # define RCU_FANOUT 64
+ 6 # else
+ 7 # define RCU_FANOUT 32
+ 8 # endif
+ 9 #endif
+10
+11 #ifdef CONFIG_RCU_FANOUT_LEAF
+12 #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
+13 #else
+14 # ifdef CONFIG_64BIT
+15 # define RCU_FANOUT_LEAF 64
+16 # else
+17 # define RCU_FANOUT_LEAF 32
+18 # endif
+19 #endif
+20
+21 #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
+22 #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
+23 #define RCU_FANOUT_3 (RCU_FANOUT_2 * RCU_FANOUT)
+24 #define RCU_FANOUT_4 (RCU_FANOUT_3 * RCU_FANOUT)
+25
+26 #if NR_CPUS <= RCU_FANOUT_1
+27 # define RCU_NUM_LVLS 1
+28 # define NUM_RCU_LVL_0 1
+29 # define NUM_RCU_NODES NUM_RCU_LVL_0
+30 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0 }
+31 # define RCU_NODE_NAME_INIT { "rcu_node_0" }
+32 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0" }
+33 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0" }
+34 #elif NR_CPUS <= RCU_FANOUT_2
+35 # define RCU_NUM_LVLS 2
+36 # define NUM_RCU_LVL_0 1
+37 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+38 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1)
+39 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1 }
+40 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1" }
+41 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1" }
+42 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1" }
+43 #elif NR_CPUS <= RCU_FANOUT_3
+44 # define RCU_NUM_LVLS 3
+45 # define NUM_RCU_LVL_0 1
+46 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+47 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+48 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2)
+49 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2 }
+50 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2" }
+51 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2" }
+52 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2" }
+53 #elif NR_CPUS <= RCU_FANOUT_4
+54 # define RCU_NUM_LVLS 4
+55 # define NUM_RCU_LVL_0 1
+56 # define NUM_RCU_LVL_1 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_3)
+57 # define NUM_RCU_LVL_2 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_2)
+58 # define NUM_RCU_LVL_3 DIV_ROUND_UP(NR_CPUS, RCU_FANOUT_1)
+59 # define NUM_RCU_NODES (NUM_RCU_LVL_0 + NUM_RCU_LVL_1 + NUM_RCU_LVL_2 + NUM_RCU_LVL_3)
+60 # define NUM_RCU_LVL_INIT { NUM_RCU_LVL_0, NUM_RCU_LVL_1, NUM_RCU_LVL_2, NUM_RCU_LVL_3 }
+61 # define RCU_NODE_NAME_INIT { "rcu_node_0", "rcu_node_1", "rcu_node_2", "rcu_node_3" }
+62 # define RCU_FQS_NAME_INIT { "rcu_node_fqs_0", "rcu_node_fqs_1", "rcu_node_fqs_2", "rcu_node_fqs_3" }
+63 # define RCU_EXP_NAME_INIT { "rcu_node_exp_0", "rcu_node_exp_1", "rcu_node_exp_2", "rcu_node_exp_3" }
+64 #else
+65 # error "CONFIG_RCU_FANOUT insufficient for NR_CPUS"
+66 #endif
+</pre>
+
+<p>The maximum number of levels in the <tt>rcu_node</tt> structure
+is currently limited to four, as specified by lines 21-24
+and the structure of the subsequent “if” statement.
+For 32-bit systems, this allows 16*32*32*32=524,288 CPUs, which
+should be sufficient for the next few years at least.
+For 64-bit systems, 16*64*64*64=4,194,304 CPUs is allowed, which
+should see us through the next decade or so.
+This four-level tree also allows kernels built with
+<tt>CONFIG_RCU_FANOUT=8</tt> to support up to 4096 CPUs,
+which might be useful in very large systems having eight CPUs per
+socket (but please note that no one has yet shown any measurable
+performance degradation due to misaligned socket and <tt>rcu_node</tt>
+boundaries).
+In addition, building kernels with a full four levels of <tt>rcu_node</tt>
+tree permits better testing of RCU's combining-tree code.
+
+</p><p>The <tt>RCU_FANOUT</tt> symbol controls how many children
+are permitted at each non-leaf level of the <tt>rcu_node</tt> tree.
+If the <tt>CONFIG_RCU_FANOUT</tt> Kconfig option is not specified,
+it is set based on the word size of the system, which is also
+the Kconfig default.
+
+</p><p>The <tt>RCU_FANOUT_LEAF</tt> symbol controls how many CPUs are
+handled by each leaf <tt>rcu_node</tt> structure.
+Experience has shown that allowing a given leaf <tt>rcu_node</tt>
+structure to handle 64 CPUs, as permitted by the number of bits in
+the <tt>->qsmask</tt> field on a 64-bit system, results in
+excessive contention for the leaf <tt>rcu_node</tt> structures'
+<tt>->lock</tt> fields.
+The number of CPUs per leaf <tt>rcu_node</tt> structure is therefore
+limited to 16 given the default value of <tt>CONFIG_RCU_FANOUT_LEAF</tt>.
+If <tt>CONFIG_RCU_FANOUT_LEAF</tt> is unspecified, the value
+selected is based on the word size of the system, just as for
+<tt>CONFIG_RCU_FANOUT</tt>.
+Lines 11-19 perform this computation.
+
+</p><p>Lines 21-24 compute the maximum number of CPUs supported by
+a single-level (which contains a single <tt>rcu_node</tt> structure),
+two-level, three-level, and four-level <tt>rcu_node</tt> tree,
+respectively, given the fanout specified by <tt>RCU_FANOUT</tt>
+and <tt>RCU_FANOUT_LEAF</tt>.
+These numbers of CPUs are retained in the
+<tt>RCU_FANOUT_1</tt>,
+<tt>RCU_FANOUT_2</tt>,
+<tt>RCU_FANOUT_3</tt>, and
+<tt>RCU_FANOUT_4</tt>
+C-preprocessor variables, respectively.
+
+</p><p>These variables are used to control the C-preprocessor <tt>#if</tt>
+statement spanning lines 26-66 that computes the number of
+<tt>rcu_node</tt> structures required for each level of the tree,
+as well as the number of levels required.
+The number of levels is placed in the <tt>NUM_RCU_LVLS</tt>
+C-preprocessor variable by lines 27, 35, 44, and 54.
+The number of <tt>rcu_node</tt> structures for the topmost level
+of the tree is always exactly one, and this value is unconditionally
+placed into <tt>NUM_RCU_LVL_0</tt> by lines 28, 36, 45, and 55.
+The rest of the levels (if any) of the <tt>rcu_node</tt> tree
+are computed by dividing the maximum number of CPUs by the
+fanout supported by the number of levels from the current level down,
+rounding up. This computation is performed by lines 37,
+46-47, and 56-58.
+Lines 31-33, 40-42, 50-52, and 62-63 create initializers
+for lockdep lock-class names.
+Finally, lines 64-66 produce an error if the maximum number of
+CPUs is too large for the specified fanout.
+
+<h3><a name="The rcu_data Structure">
+The <tt>rcu_data</tt> Structure</a></h3>
+
+<p>The <tt>rcu_data</tt> maintains the per-CPU state for the
+corresponding flavor of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+This structure is the
+focus of quiescent-state detection and RCU callback queuing.
+It also tracks its relationship to the corresponding leaf
+<tt>rcu_node</tt> structure to allow more-efficient
+propagation of quiescent states up the <tt>rcu_node</tt>
+combining tree.
+Like the <tt>rcu_node</tt> structure, it provides a local
+copy of the grace-period information to allow for-free
+synchronized
+access to this information from the corresponding CPU.
+Finally, this structure records past dyntick-idle state
+for the corresponding CPU and also tracks statistics.
+
+</p><p>The <tt>rcu_data</tt> structure's fields are discussed,
+singly and in groups, in the following sections.
+
+<h5>Connection to Other Data Structures</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 int cpu;
+ 2 struct rcu_state *rsp;
+ 3 struct rcu_node *mynode;
+ 4 struct rcu_dynticks *dynticks;
+ 5 unsigned long grpmask;
+ 6 bool beenonline;
+</pre>
+
+<p>The <tt>->cpu</tt> field contains the number of the
+corresponding CPU, the <tt>->rsp</tt> pointer references
+the corresponding <tt>rcu_state</tt> structure (and is most frequently
+used to locate the name of the corresponding flavor of RCU for tracing),
+and the <tt>->mynode</tt> field references the corresponding
+<tt>rcu_node</tt> structure.
+The <tt>->mynode</tt> is used to propagate quiescent states
+up the combining tree.
+<p>The <tt>->dynticks</tt> pointer references the
+<tt>rcu_dynticks</tt> structure corresponding to this
+CPU.
+Recall that a single per-CPU instance of the <tt>rcu_dynticks</tt>
+structure is shared among all flavors of RCU.
+These first four fields are constant and therefore require not
+synchronization.
+
+</p><p>The <tt>->grpmask</tt> field indicates the bit in
+the <tt>->mynode->qsmask</tt> corresponding to this
+<tt>rcu_data</tt> structure, and is also used when propagating
+quiescent states.
+The <tt>->beenonline</tt> flag is set whenever the corresponding
+CPU comes online, which means that the debugfs tracing need not dump
+out any <tt>rcu_data</tt> structure for which this flag is not set.
+
+<h5>Quiescent-State and Grace-Period Tracking</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 unsigned long completed;
+ 2 unsigned long gpnum;
+ 3 bool cpu_no_qs;
+ 4 bool core_needs_qs;
+ 5 bool gpwrap;
+ 6 unsigned long rcu_qs_ctr_snap;
+</pre>
+
+<p>The <tt>completed</tt> and <tt>gpnum</tt>
+fields are the counterparts of the fields of the same name
+in the <tt>rcu_state</tt> and <tt>rcu_node</tt> structures.
+They may each lag up to one behind their <tt>rcu_node</tt>
+counterparts, but in <tt>CONFIG_NO_HZ_IDLE</tt> and
+<tt>CONFIG_NO_HZ_FULL</tt> kernels can lag
+arbitrarily far behind for CPUs in dyntick-idle mode (but these counters
+will catch up upon exit from dyntick-idle mode).
+If a given <tt>rcu_data</tt> structure's <tt>->gpnum</tt> and
+<tt>->complete</tt> fields are equal, then this <tt>rcu_data</tt>
+structure believes that RCU is idle.
+Otherwise, as with the <tt>rcu_state</tt> and <tt>rcu_node</tt>
+structure,
+the <tt>->gpnum</tt> field will be one greater than the
+<tt>->complete</tt> fields, with <tt>->gpnum</tt>
+indicating which grace period this <tt>rcu_data</tt> believes
+is still being waited for.
+
+<p>@@QQ@@
+All this replication of the grace period numbers can only cause
+massive confusion.
+Why not just keep a global pair of counters and be done with it???
+<p>@@QQA@@
+Because if there was only a single global pair of grace-period numbers,
+there would need to be a single global lock to allow safely accessing
+and updating them.
+And if we are not going to have a single global lock, we need to carefully
+manage the numbers on a per-node basis.
+Recall from the answer to a previous Quick Quiz that the consequences
+of applying a previously sampled quiescent state to the wrong
+grace period are quite severe.
+<p>@@QQE@@
+
+</p><p>The <tt>->cpu_no_qs</tt> flag indicates that the
+CPU has not yet passed through a quiescent state,
+while the <tt>->core_needs_qs</tt> flag indicates that the
+RCU core needs a quiescent state from the corresponding CPU.
+The <tt>->gpwrap</tt> field indicates that the corresponding
+CPU has remained idle for so long that the <tt>completed</tt>
+and <tt>gpnum</tt> counters are in danger of overflow, which
+will cause the CPU to disregard the values of its counters on
+its next exit from idle.
+Finally, the <tt>rcu_qs_ctr_snap</tt> field is used to detect
+cases where a given operation has resulted in a quiescent state
+for all flavors of RCU, for example, <tt>cond_resched_rcu_qs()</tt>.
+
+<h5>RCU Callback Handling</h5>
+
+<p>In the absence of CPU-hotplug events, RCU callbacks are invoked by
+the same CPU that registered them.
+This is strictly a cache-locality optimization: callbacks can and
+do get invoked on CPUs other than the one that registered them.
+After all, if the CPU that registered a given callback has gone
+offline before the callback can be invoked, there really is no other
+choice.
+
+</p><p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 struct rcu_head *nxtlist;
+ 2 struct rcu_head **nxttail[RCU_NEXT_SIZE];
+ 3 unsigned long nxtcompleted[RCU_NEXT_SIZE];
+ 4 long qlen_lazy;
+ 5 long qlen;
+ 6 long qlen_last_fqs_check;
+ 7 unsigned long n_force_qs_snap;
+ 8 unsigned long n_cbs_invoked;
+ 9 unsigned long n_cbs_orphaned;
+10 unsigned long n_cbs_adopted;
+11 long blimit;
+</pre>
+
+<p>The <tt>->nxtlist</tt> pointer and the
+<tt>->nxttail[]</tt> array form a four-segment list with
+older callbacks near the head and newer ones near the tail.
+Each segment contains callbacks with the corresponding relationship
+to the current grace period.
+The pointer out of the end of each of the four segments is referenced
+by the element of the <tt>->nxttail[]</tt> array indexed by
+<tt>RCU_DONE_TAIL</tt> (for callbacks handled by a prior grace period),
+<tt>RCU_WAIT_TAIL</tt> (for callbacks waiting on the current grace period),
+<tt>RCU_NEXT_READY_TAIL</tt> (for callbacks that will wait on the next
+grace period), and
+<tt>RCU_NEXT_TAIL</tt> (for callbacks that are not yet associated
+with a specific grace period)
+respectively, as shown in the following figure.
+
+</p><p><img src="nxtlist.svg" alt="nxtlist.svg" width="40%">
+
+</p><p>In this figure, the <tt>->nxtlist</tt> pointer references the
+first
+RCU callback in the list.
+The <tt>->nxttail[RCU_DONE_TAIL]</tt> array element references
+the <tt>->nxtlist</tt> pointer itself, indicating that none
+of the callbacks is ready to invoke.
+The <tt>->nxttail[RCU_WAIT_TAIL]</tt> array element references callback
+CB 2's <tt>->next</tt> pointer, which indicates that
+CB 1 and CB 2 are both waiting on the current grace period.
+The <tt>->nxttail[RCU_NEXT_READY_TAIL]</tt> array element
+references the same RCU callback that <tt>->nxttail[RCU_WAIT_TAIL]</tt>
+does, which indicates that there are no callbacks waiting on the next
+RCU grace period.
+The <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element references
+CB 4's <tt>->next</tt> pointer, indicating that all the
+remaining RCU callbacks have not yet been assigned to an RCU grace
+period.
+Note that the <tt>->nxttail[RCU_NEXT_TAIL]</tt> array element
+always references the last RCU callback's <tt>->next</tt> pointer
+unless the callback list is empty, in which case it references
+the <tt>->nxtlist</tt> pointer.
+
+</p><p>CPUs advance their callbacks from the
+<tt>RCU_NEXT_TAIL</tt> to the <tt>RCU_NEXT_READY_TAIL</tt> to the
+<tt>RCU_WAIT_TAIL</tt> to the <tt>RCU_DONE_TAIL</tt> list segments
+as grace periods advance.
+The CPU advances the callbacks in its <tt>rcu_data</tt> structure
+whenever it notices that another RCU grace period has completed.
+The CPU detects the completion of an RCU grace period by noticing
+that the value of its <tt>rcu_data</tt> structure's
+<tt>->completed</tt> field differs from that of its leaf
+<tt>rcu_node</tt> structure.
+Recall that each <tt>rcu_node</tt> structure's
+<tt>->completed</tt> field is updated at the end of each
+grace period.
+
+</p><p>The <tt>->nxtcompleted[]</tt> array records grace-period
+numbers corresponding to the list segments.
+This allows CPUs that go idle for extended periods to determine
+which of their callbacks are ready to be invoked after reawakening.
+
+</p><p>The <tt>->qlen</tt> counter contains the number of
+callbacks in <tt>->nxtlist</tt>, and the
+<tt>->qlen_lazy</tt> contains the number of those callbacks that
+are known to only free memory, and whose invocation can therefore
+be safely deferred.
+The <tt>->qlen_last_fqs_check</tt> and
+<tt>->n_force_qs_snap</tt> coordinate the forcing of quiescent
+states from <tt>call_rcu()</tt> and friends when callback
+lists grow excessively long.
+
+</p><p>The <tt>->n_cbs_invoked</tt>,
+<tt>->n_cbs_orphaned</tt>, and <tt>->n_cbs_adopted</tt>
+fields count the number of callbacks invoked,
+sent to other CPUs when this CPU goes offline,
+and received from other CPUs when those other CPUs go offline.
+Finally, the <tt>->blimit</tt> counter is the maximum number of
+RCU callbacks that may be invoked at a given time.
+
+<h5>Dyntick-Idle Handling</h5>
+
+<p>This portion of the <tt>rcu_data</tt> structure is declared
+as follows:
+
+<pre>
+ 1 int dynticks_snap;
+ 2 unsigned long dynticks_fqs;
+</pre>
+
+The <tt>->dynticks_snap</tt> field is used to take a snapshot
+of the corresponding CPU's dyntick-idle state when forcing
+quiescent states, and is therefore accessed from other CPUs.
+Finally, the <tt>->dynticks_fqs</tt> field is used to
+count the number of times this CPU is determined to be in
+dyntick-idle state, and is used for tracing and debugging purposes.
+
+<h3><a name="The rcu_dynticks Structure">
+The <tt>rcu_dynticks</tt> Structure</a></h3>
+
+<p>The <tt>rcu_dynticks</tt> maintains the per-CPU dyntick-idle state
+for the corresponding CPU.
+Unlike the other structures, <tt>rcu_dynticks</tt> is not
+replicated over the different flavors of RCU.
+The fields in this structure may be accessed only from the corresponding
+CPU (and from tracing) unless otherwise stated.
+Its fields are as follows:
+
+<pre>
+ 1 int dynticks_nesting;
+ 2 int dynticks_nmi_nesting;
+ 3 atomic_t dynticks;
+</pre>
+
+<p>The <tt>->dynticks_nesting</tt> field counts the
+nesting depth of normal interrupts.
+In addition, this counter is incremented when exiting dyntick-idle
+mode and decremented when entering it.
+This counter can therefore be thought of as counting the number
+of reasons why this CPU cannot be permitted to enter dyntick-idle
+mode, aside from non-maskable interrupts (NMIs).
+NMIs are counted by the <tt>->dynticks_nmi_nesting</tt>
+field, except that NMIs that interrupt non-dyntick-idle execution
+are not counted.
+
+</p><p>Finally, the <tt>->dynticks</tt> field counts the corresponding
+CPU's transitions to and from dyntick-idle mode, so that this counter
+has an even value when the CPU is in dyntick-idle mode and an odd
+value otherwise.
+
+<p>@@QQ@@
+Why not just count all NMIs?
+Wouldn't that be simpler and less error prone?
+<p>@@QQA@@
+It seems simpler only until you think hard about how to go about
+updating the <tt>rcu_dynticks</tt> structure's
+<tt>->dynticks</tt> field.
+<p>@@QQE@@
+
+</p><p>Additional fields are present for some special-purpose
+builds, and are discussed separately.
+
+<h3><a name="The rcu_head Structure">
+The <tt>rcu_head</tt> Structure</a></h3>
+
+<p>Each <tt>rcu_head</tt> structure represents an RCU callback.
+These structures are normally embedded within RCU-protected data
+structures whose algorithms use asynchronous grace periods.
+In contrast, when using algorithms that block waiting for RCU grace periods,
+RCU users need not provide <tt>rcu_head</tt> structures.
+
+</p><p>The <tt>rcu_head</tt> structure has fields as follows:
+
+<pre>
+ 1 struct rcu_head *next;
+ 2 void (*func)(struct rcu_head *head);
+</pre>
+
+<p>The <tt>->next</tt> field is used
+to link the <tt>rcu_head</tt> structures together in the
+lists within the <tt>rcu_data</tt> structures.
+The <tt>->func</tt> field is a pointer to the function
+to be called when the callback is ready to be invoked, and
+this function is passed a pointer to the <tt>rcu_head</tt>
+structure.
+However, <tt>kfree_rcu()</tt> uses the <tt>->func</tt>
+field to record the offset of the <tt>rcu_head</tt>
+structure within the enclosing RCU-protected data structure.
+
+</p><p>Both of these fields are used internally by RCU.
+From the viewpoint of RCU users, this structure is an
+opaque “cookie”.
+
+<p>@@QQ@@
+Given that the callback function <tt>->func</tt>
+is passed a pointer to the <tt>rcu_head</tt> structure,
+how is that function supposed to find the beginning of the
+enclosing RCU-protected data structure?
+<p>@@QQA@@
+In actual practice, there is a separate callback function per
+type of RCU-protected data structure.
+The callback function can therefore use the <tt>container_of()</tt>
+macro in the Linux kernel (or other pointer-manipulation facilities
+in other software environments) to find the beginning of the
+enclosing structure.
+<p>@@QQE@@
+
+<h3><a name="RCU-Specific Fields in the task_struct Structure">
+RCU-Specific Fields in the <tt>task_struct</tt> Structure</a></h3>
+
+<p>The <tt>CONFIG_TREE_PREEMPT_RCU</tt> implementation uses some
+additional fields in the <tt>task_struct</tt> structure:
+
+<pre>
+ 1 #ifdef CONFIG_PREEMPT_RCU
+ 2 int rcu_read_lock_nesting;
+ 3 union rcu_special rcu_read_unlock_special;
+ 4 struct list_head rcu_node_entry;
+ 5 struct rcu_node *rcu_blocked_node;
+ 6 #endif /* #ifdef CONFIG_PREEMPT_RCU */
+ 7 #ifdef CONFIG_TASKS_RCU
+ 8 unsigned long rcu_tasks_nvcsw;
+ 9 bool rcu_tasks_holdout;
+10 struct list_head rcu_tasks_holdout_list;
+11 int rcu_tasks_idle_cpu;
+12 #endif /* #ifdef CONFIG_TASKS_RCU */
+</pre>
+
+<p>The <tt>->rcu_read_lock_nesting</tt> field records the
+nesting level for RCU read-side critical sections, and
+the <tt>->rcu_read_unlock_special</tt> field is a bitmask
+that records special conditions that require <tt>rcu_read_unlock()</tt>
+to do additional work.
+The <tt>->rcu_node_entry</tt> field is used to form lists of
+tasks that have blocked within preemptible-RCU read-side critical
+sections and the <tt>->rcu_blocked_node</tt> field references
+the <tt>rcu_node</tt> structure whose list this task is a member of,
+or <tt>NULL</tt> if it is not blocked within a preemptible-RCU
+read-side critical section.
+
+<p>The <tt>->rcu_tasks_nvcsw</tt> field tracks the number of
+voluntary context switches that this task had undergone at the
+beginning of the current tasks-RCU grace period,
+<tt>->rcu_tasks_holdout</tt> is set if the current tasks-RCU
+grace period is waiting on this task, <tt>->rcu_tasks_holdout_list</tt>
+is a list element enqueuing this task on the holdout list,
+and <tt>->rcu_tasks_idle_cpu</tt> tracks which CPU this
+idle task is running, but only if the task is currently running,
+that is, if the CPU is currently idle.
+
+<p>@@QQ@@
+Why is <tt>->rcu_boosted</tt> required, given that there is
+a <tt>RCU_READ_UNLOCK_BOOSTED</tt> bit in
+<tt>->rcu_read_unlock_special</tt>?
+<p>@@QQA@@
+The <tt>->rcu_read_unlock_special</tt> field may only be
+updated by the task itself.
+By definition, RCU priority boosting must be carried out by some
+other task.
+This other task cannot safely update the boosted task's
+<tt>->rcu_read_unlock_special</tt> field without the use of
+expensive atomic instructions.
+The <tt>->rcu_boosted</tt> field is therefore used by the
+boosting task to let the boosted task know that it has been boosted.
+The boosted task makes use of the
+<tt>RCU_READ_UNLOCK_BOOSTED</tt> bit in
+<tt>->rcu_read_unlock_special</tt>
+when deboosting itself.
+<p>@@QQE@@
+
+<h3><a name="Accessor Functions">
+Accessor Functions</a></h3>
+
+<p>The following listing shows the
+<tt>rcu_get_root()</tt>, <tt>rcu_for_each_node_breadth_first</tt>,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt>, and
+<tt>rcu_for_each_leaf_node()</tt> function and macros:
+
+<pre>
+ 1 static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+ 2 {
+ 3 return &rsp->node[0];
+ 4 }
+ 5
+ 6 #define rcu_for_each_node_breadth_first(rsp, rnp) \
+ 7 for ((rnp) = &(rsp)->node[0]; \
+ 8 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+ 9
+ 10 #define rcu_for_each_nonleaf_node_breadth_first(rsp, rnp) \
+ 11 for ((rnp) = &(rsp)->node[0]; \
+ 12 (rnp) < (rsp)->level[NUM_RCU_LVLS - 1]; (rnp)++)
+ 13
+ 14 #define rcu_for_each_leaf_node(rsp, rnp) \
+ 15 for ((rnp) = (rsp)->level[NUM_RCU_LVLS - 1]; \
+ 16 (rnp) < &(rsp)->node[NUM_RCU_NODES]; (rnp)++)
+</pre>
+
+<p>The <tt>rcu_get_root()</tt> simply returns a pointer to the
+first element of the specified <tt>rcu_state</tt> structure's
+<tt>->node[]</tt> array, which is the root <tt>rcu_node</tt>
+structure.
+
+</p><p>As noted earlier, the <tt>rcu_for_each_node_breadth_first()</tt>
+macro takes advantage of the layout of the <tt>rcu_node</tt>
+structures in the <tt>rcu_state</tt> structure's
+<tt>->node[]</tt> array, performing a breadth-first traversal by
+simply traversing the array in order.
+The <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> macro operates
+similarly, but traverses only the first part of the array, thus excluding
+the leaf <tt>rcu_node</tt> structures.
+Finally, the <tt>rcu_for_each_leaf_node()</tt> macro traverses only
+the last part of the array, thus traversing only the leaf
+<tt>rcu_node</tt> structures.
+
+<p>@@QQ@@
+What do <tt>rcu_for_each_nonleaf_node_breadth_first()</tt> and
+<tt>rcu_for_each_leaf_node()</tt> do if the <tt>rcu_node</tt> tree
+contains only a single node?
+<p>@@QQA@@
+In the single-node case,
+<tt>rcu_for_each_nonleaf_node_breadth_first()</tt> is a no-op
+and <tt>rcu_for_each_leaf_node()</tt> traverses the single node.
+<p>@@QQE@@
+
+<h3><a name="Summary">
+Summary</a></h3>
+
+So each flavor of RCU is represented by an <tt>rcu_state</tt> structure,
+which contains a combining tree of <tt>rcu_node</tt> and
+<tt>rcu_data</tt> structures.
+Finally, in <tt>CONFIG_NO_HZ_IDLE</tt> kernels, each CPU's dyntick-idle
+state is tracked by an <tt>rcu_dynticks</tt> structure.
+
+If you made it this far, you are well prepared to read the code
+walkthroughs in the other articles in this series.
+
+<h3><a name="Acknowledgments">
+Acknowledgments</a></h3>
+
+I owe thanks to Cyrill Gorcunov, Mathieu Desnoyers, Dhaval Giani, Paul
+Turner, Abhishek Srivastava, Matt Kowalczyk, and Serge Hallyn
+for helping me get this document into a more human-readable state.
+
+<h3><a name="Legal Statement">
+Legal Statement</a></h3>
+
+<p>This work represents the view of the author and does not necessarily
+represent the view of IBM.
+
+</p><p>Linux is a registered trademark of Linus Torvalds.
+
+</p><p>Other company, product, and service names may be trademarks or
+service marks of others.
+
+
+<p>@@QQAL@@
+
+
+</body></html>
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2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier()
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (8 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity Paul E. McKenney
` (3 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
This commit adds a Quick Quiz whose answer explains why the compiler
code reordering enabled by CONFIG_PREEMPT=n's empty rcu_read_lock()
and rcu_read_unlock() functions does not hinder RCU's ability to figure
out which RCU read-side critical sections have completed and not.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
.../RCU/Design/Requirements/Requirements.html | 130 ++++++++++++++-------
.../RCU/Design/Requirements/Requirements.htmlx | 28 +++++
2 files changed, 115 insertions(+), 43 deletions(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 59acd82e67d4..2a56031bfdd4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -583,6 +583,17 @@ The first and second guarantees require unbelievably strict ordering!
Are all these memory barriers <i> really</i> required?
<br><a href="#qq6answer">Answer</a>
+<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
+You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+generate absolutely no code in some kernel builds.
+This means that the compiler might arbitrarily rearrange consecutive
+RCU read-side critical sections.
+Given such rearrangement, if a given RCU read-side critical section
+is done, how can you be sure that all prior RCU read-side critical
+sections are done?
+Won't the compiler rearrangements make that impossible to determine?
+<br><a href="#qq7answer">Answer</a>
+
<p>
Note that these memory-barrier requirements do not replace the fundamental
RCU requirement that a grace period wait for all pre-existing readers.
@@ -626,9 +637,9 @@ inconvenience can be avoided through use of the
<tt>call_rcu()</tt> and <tt>kfree_rcu()</tt> API members
described later in this document.
-<p><a name="Quick Quiz 7"><b>Quick Quiz 7</b>:</a>
+<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
But how does the upgrade-to-write operation exclude other readers?
-<br><a href="#qq7answer">Answer</a>
+<br><a href="#qq8answer">Answer</a>
<p>
This guarantee allows lookup code to be shared between read-side
@@ -714,9 +725,9 @@ to do significant reordering.
This is by design: Any significant ordering constraints would slow down
these fast-path APIs.
-<p><a name="Quick Quiz 8"><b>Quick Quiz 8</b>:</a>
+<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
Can't the compiler also reorder this code?
-<br><a href="#qq8answer">Answer</a>
+<br><a href="#qq9answer">Answer</a>
<h3><a name="Readers Do Not Exclude Updaters">Readers Do Not Exclude Updaters</a></h3>
@@ -769,10 +780,10 @@ new readers can start immediately after <tt>synchronize_rcu()</tt>
starts, and <tt>synchronize_rcu()</tt> is under no
obligation to wait for these new readers.
-<p><a name="Quick Quiz 9"><b>Quick Quiz 9</b>:</a>
+<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a>
Suppose that synchronize_rcu() did wait until all readers had completed.
Would the updater be able to rely on this?
-<br><a href="#qq9answer">Answer</a>
+<br><a href="#qq10answer">Answer</a>
<h3><a name="Grace Periods Don't Partition Read-Side Critical Sections">
Grace Periods Don't Partition Read-Side Critical Sections</a></h3>
@@ -969,11 +980,11 @@ grace period.
As a result, an RCU read-side critical section cannot partition a pair
of RCU grace periods.
-<p><a name="Quick Quiz 10"><b>Quick Quiz 10</b>:</a>
+<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a>
How long a sequence of grace periods, each separated by an RCU read-side
critical section, would be required to partition the RCU read-side
critical sections at the beginning and end of the chain?
-<br><a href="#qq10answer">Answer</a>
+<br><a href="#qq11answer">Answer</a>
<h3><a name="Disabling Preemption Does Not Block Grace Periods">
Disabling Preemption Does Not Block Grace Periods</a></h3>
@@ -1127,9 +1138,9 @@ synchronization primitives be legal within RCU read-side critical sections,
including spinlocks, sequence locks, atomic operations, reference
counters, and memory barriers.
-<p><a name="Quick Quiz 11"><b>Quick Quiz 11</b>:</a>
+<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a>
What about sleeping locks?
-<br><a href="#qq11answer">Answer</a>
+<br><a href="#qq12answer">Answer</a>
<p>
It often comes as a surprise that many algorithms do not require a
@@ -1354,12 +1365,12 @@ write an RCU callback function that takes too long.
Long-running operations should be relegated to separate threads or
(in the Linux kernel) workqueues.
-<p><a name="Quick Quiz 12"><b>Quick Quiz 12</b>:</a>
+<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a>
Why does line 19 use <tt>rcu_access_pointer()</tt>?
After all, <tt>call_rcu()</tt> on line 25 stores into the
structure, which would interact badly with concurrent insertions.
Doesn't this mean that <tt>rcu_dereference()</tt> is required?
-<br><a href="#qq12answer">Answer</a>
+<br><a href="#qq13answer">Answer</a>
<p>
However, all that <tt>remove_gp_cb()</tt> is doing is
@@ -1406,14 +1417,14 @@ This was due to the fact that RCU was not heavily used within DYNIX/ptx,
so the very few places that needed something like
<tt>synchronize_rcu()</tt> simply open-coded it.
-<p><a name="Quick Quiz 13"><b>Quick Quiz 13</b>:</a>
+<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a>
Earlier it was claimed that <tt>call_rcu()</tt> and
<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
by readers.
But how can that be correct, given that the invocation of the callback
and the freeing of the memory (respectively) must still wait for
a grace period to elapse?
-<br><a href="#qq13answer">Answer</a>
+<br><a href="#qq14answer">Answer</a>
<p>
But what if the updater must wait for the completion of code to be
@@ -1838,11 +1849,11 @@ kthreads to be spawned.
Therefore, invoking <tt>synchronize_rcu()</tt> during scheduler
initialization can result in deadlock.
-<p><a name="Quick Quiz 14"><b>Quick Quiz 14</b>:</a>
+<p><a name="Quick Quiz 15"><b>Quick Quiz 15</b>:</a>
So what happens with <tt>synchronize_rcu()</tt> during
scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
kernels?
-<br><a href="#qq14answer">Answer</a>
+<br><a href="#qq15answer">Answer</a>
<p>
I learned of these boot-time requirements as a result of a series of
@@ -2547,10 +2558,10 @@ If you needed to wait on multiple different flavors of SRCU
(but why???), you would need to create a wrapper function resembling
<tt>call_my_srcu()</tt> for each SRCU flavor.
-<p><a name="Quick Quiz 15"><b>Quick Quiz 15</b>:</a>
+<p><a name="Quick Quiz 16"><b>Quick Quiz 16</b>:</a>
But what if I need to wait for multiple RCU flavors, but I also need
the grace periods to be expedited?
-<br><a href="#qq15answer">Answer</a>
+<br><a href="#qq16answer">Answer</a>
<p>
Again, it is usually better to adjust the RCU read-side critical sections
@@ -2827,18 +2838,51 @@ adhered to the as-if rule than it is to actually adhere to it!
<a name="qq7answer"></a>
<p><b>Quick Quiz 7</b>:
-But how does the upgrade-to-write operation exclude other readers?
+You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+generate absolutely no code in some kernel builds.
+This means that the compiler might arbitrarily rearrange consecutive
+RCU read-side critical sections.
+Given such rearrangement, if a given RCU read-side critical section
+is done, how can you be sure that all prior RCU read-side critical
+sections are done?
+Won't the compiler rearrangements make that impossible to determine?
</p><p><b>Answer</b>:
-It doesn't, just like normal RCU updates, which also do not exclude
-RCU readers.
+In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+generate absolutely no code, RCU infers quiescent states only at
+special locations, for example, within the scheduler.
+Because calls to <tt>schedule()</tt> had better prevent calling-code
+accesses to shared variables from being rearranged across the call to
+<tt>schedule()</tt>, if RCU detects the end of a given RCU read-side
+critical section, it will necessarily detect the end of all prior
+RCU read-side critical sections, no matter how aggressively the
+compiler scrambles the code.
+
+<p>
+Again, this all assumes that the compiler cannot scramble code across
+calls to the scheduler, out of interrupt handlers, into the idle loop,
+into user-mode code, and so on.
+But if your kernel build allows that sort of scrambling, you have broken
+far more than just RCU!
</p><p><a href="#Quick%20Quiz%207"><b>Back to Quick Quiz 7</b>.</a>
<a name="qq8answer"></a>
<p><b>Quick Quiz 8</b>:
+But how does the upgrade-to-write operation exclude other readers?
+
+
+</p><p><b>Answer</b>:
+It doesn't, just like normal RCU updates, which also do not exclude
+RCU readers.
+
+
+</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
+
+<a name="qq9answer"></a>
+<p><b>Quick Quiz 9</b>:
Can't the compiler also reorder this code?
@@ -2848,10 +2892,10 @@ No, the volatile casts in <tt>READ_ONCE()</tt> and
this particular case.
-</p><p><a href="#Quick%20Quiz%208"><b>Back to Quick Quiz 8</b>.</a>
+</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
-<a name="qq9answer"></a>
-<p><b>Quick Quiz 9</b>:
+<a name="qq10answer"></a>
+<p><b>Quick Quiz 10</b>:
Suppose that synchronize_rcu() did wait until all readers had completed.
Would the updater be able to rely on this?
@@ -2866,10 +2910,10 @@ Therefore, the code following
in any case.
-</p><p><a href="#Quick%20Quiz%209"><b>Back to Quick Quiz 9</b>.</a>
+</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a>
-<a name="qq10answer"></a>
-<p><b>Quick Quiz 10</b>:
+<a name="qq11answer"></a>
+<p><b>Quick Quiz 11</b>:
How long a sequence of grace periods, each separated by an RCU read-side
critical section, would be required to partition the RCU read-side
critical sections at the beginning and end of the chain?
@@ -2883,10 +2927,10 @@ Therefore, even in practice, RCU users must abide by the theoretical rather
than the practical answer.
-</p><p><a href="#Quick%20Quiz%2010"><b>Back to Quick Quiz 10</b>.</a>
+</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a>
-<a name="qq11answer"></a>
-<p><b>Quick Quiz 11</b>:
+<a name="qq12answer"></a>
+<p><b>Quick Quiz 12</b>:
What about sleeping locks?
@@ -2914,10 +2958,10 @@ the mutex was not immediately available.
Either way, <tt>mutex_trylock()</tt> returns immediately without sleeping.
-</p><p><a href="#Quick%20Quiz%2011"><b>Back to Quick Quiz 11</b>.</a>
+</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a>
-<a name="qq12answer"></a>
-<p><b>Quick Quiz 12</b>:
+<a name="qq13answer"></a>
+<p><b>Quick Quiz 13</b>:
Why does line 19 use <tt>rcu_access_pointer()</tt>?
After all, <tt>call_rcu()</tt> on line 25 stores into the
structure, which would interact badly with concurrent insertions.
@@ -2933,10 +2977,10 @@ is released on line 25, which in turn means that
<tt>rcu_access_pointer()</tt> suffices.
-</p><p><a href="#Quick%20Quiz%2012"><b>Back to Quick Quiz 12</b>.</a>
+</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a>
-<a name="qq13answer"></a>
-<p><b>Quick Quiz 13</b>:
+<a name="qq14answer"></a>
+<p><b>Quick Quiz 14</b>:
Earlier it was claimed that <tt>call_rcu()</tt> and
<tt>kfree_rcu()</tt> allowed updaters to avoid being blocked
by readers.
@@ -2957,10 +3001,10 @@ next update as soon as it has invoked <tt>call_rcu()</tt> or
grace period.
-</p><p><a href="#Quick%20Quiz%2013"><b>Back to Quick Quiz 13</b>.</a>
+</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
-<a name="qq14answer"></a>
-<p><b>Quick Quiz 14</b>:
+<a name="qq15answer"></a>
+<p><b>Quick Quiz 15</b>:
So what happens with <tt>synchronize_rcu()</tt> during
scheduler initialization for <tt>CONFIG_PREEMPT=n</tt>
kernels?
@@ -2976,10 +3020,10 @@ so it is still necessary to avoid invoking <tt>synchronize_rcu()</tt>
during scheduler initialization.
-</p><p><a href="#Quick%20Quiz%2014"><b>Back to Quick Quiz 14</b>.</a>
+</p><p><a href="#Quick%20Quiz%2015"><b>Back to Quick Quiz 15</b>.</a>
-<a name="qq15answer"></a>
-<p><b>Quick Quiz 15</b>:
+<a name="qq16answer"></a>
+<p><b>Quick Quiz 16</b>:
But what if I need to wait for multiple RCU flavors, but I also need
the grace periods to be expedited?
@@ -2991,7 +3035,7 @@ But if that is nevertheless a problem, you can use workqueues or multiple
kthreads to wait on the various expedited grace periods concurrently.
-</p><p><a href="#Quick%20Quiz%2015"><b>Back to Quick Quiz 15</b>.</a>
+</p><p><a href="#Quick%20Quiz%2016"><b>Back to Quick Quiz 16</b>.</a>
</body></html>
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 6ff4966672e2..98da30ca84c4 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -682,6 +682,34 @@ That said, it is much easier to fool yourself into believing that you have
adhered to the as-if rule than it is to actually adhere to it!
<p>@@QQE@@
+<p>@@QQ@@
+You claim that <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+generate absolutely no code in some kernel builds.
+This means that the compiler might arbitrarily rearrange consecutive
+RCU read-side critical sections.
+Given such rearrangement, if a given RCU read-side critical section
+is done, how can you be sure that all prior RCU read-side critical
+sections are done?
+Won't the compiler rearrangements make that impossible to determine?
+<p>@@QQA@@
+In cases where <tt>rcu_read_lock()</tt> and <tt>rcu_read_unlock()</tt>
+generate absolutely no code, RCU infers quiescent states only at
+special locations, for example, within the scheduler.
+Because calls to <tt>schedule()</tt> had better prevent calling-code
+accesses to shared variables from being rearranged across the call to
+<tt>schedule()</tt>, if RCU detects the end of a given RCU read-side
+critical section, it will necessarily detect the end of all prior
+RCU read-side critical sections, no matter how aggressively the
+compiler scrambles the code.
+
+<p>
+Again, this all assumes that the compiler cannot scramble code across
+calls to the scheduler, out of interrupt handlers, into the idle loop,
+into user-mode code, and so on.
+But if your kernel build allows that sort of scrambling, you have broken
+far more than just RCU!
+<p>@@QQE@@
+
<p>
Note that these memory-barrier requirements do not replace the fundamental
RCU requirement that a grace period wait for all pre-existing readers.
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (9 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs Paul E. McKenney
` (2 subsequent siblings)
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
The "transitivity" section mentions cumulativity in a potentially
confusing way. Contrary to the current wording, cumulativity is
not transitivity, but rather a hardware discipline that can be used
to implement transitivity on ARM and PowerPC CPUs. This commit
therefore deletes the mention of cumulativity.
Reported-by: Luc Maranget <luc.maranget@inria.fr>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 57e4a4b053c5..8367d393cba2 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1270,7 +1270,7 @@ TRANSITIVITY
Transitivity is a deeply intuitive notion about ordering that is not
always provided by real computer systems. The following example
-demonstrates transitivity (also called "cumulativity"):
+demonstrates transitivity:
CPU 1 CPU 2 CPU 3
======================= ======================= =======================
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (10 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example Paul E. McKenney
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
There is already a blanket statement about no member of RCU's API
being legal from an offline CPU, but add an explicit note where it
states that it is illegal to invoke call_rcu() from an NMI handler.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/RCU/Design/Requirements/Requirements.html | 3 ++-
Documentation/RCU/Design/Requirements/Requirements.htmlx | 3 ++-
2 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/Documentation/RCU/Design/Requirements/Requirements.html b/Documentation/RCU/Design/Requirements/Requirements.html
index 2a56031bfdd4..01e12b86e81f 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.html
+++ b/Documentation/RCU/Design/Requirements/Requirements.html
@@ -1354,7 +1354,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor
<tt>synchronize_rcu_expedited()</tt> would be legal,
including within preempt-disable code, <tt>local_bh_disable()</tt> code,
interrupt-disable code, and interrupt handlers.
-However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers
+and from offline CPUs.
The callback function (<tt>remove_gp_cb()</tt> in this case) will be
executed within softirq (software interrupt) environment within the
Linux kernel,
diff --git a/Documentation/RCU/Design/Requirements/Requirements.htmlx b/Documentation/RCU/Design/Requirements/Requirements.htmlx
index 98da30ca84c4..3355f1f9384c 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.htmlx
+++ b/Documentation/RCU/Design/Requirements/Requirements.htmlx
@@ -1513,7 +1513,8 @@ situations where neither <tt>synchronize_rcu()</tt> nor
<tt>synchronize_rcu_expedited()</tt> would be legal,
including within preempt-disable code, <tt>local_bh_disable()</tt> code,
interrupt-disable code, and interrupt handlers.
-However, even <tt>call_rcu()</tt> is illegal within NMI handlers.
+However, even <tt>call_rcu()</tt> is illegal within NMI handlers
+and from offline CPUs.
The callback function (<tt>remove_gp_cb()</tt> in this case) will be
executed within softirq (software interrupt) environment within the
Linux kernel,
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (11 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example Paul E. McKenney
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, Paul E. McKenney
This commit adds a couple of paragraphs to the description of RCU's
combining tree explaining how the combining tree keeps lock contention
acceptably low, despite RCU grace periods being global operations.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
.../Design/Data-Structures/Data-Structures.html | 23 ++++++++++++++++++++++
.../Design/Data-Structures/Data-Structures.htmlx | 23 ++++++++++++++++++++++
2 files changed, 46 insertions(+)
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.html b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
index ba9fbb5177f6..d15744b87b99 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.html
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.html
@@ -100,6 +100,29 @@ On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
as small as 2 if you wish, which would permit only 16 CPUs, which
is useful for testing.
+</p><p>This multi-level combining tree allows us to get most of the
+performance and scalability
+benefits of partitioning, even though RCU grace-period detection is
+inherently a global operation.
+The trick here is that only the last CPU to report a quiescent state
+into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
+structure at the next level up the tree.
+This means that at the leaf-level <tt>rcu_node</tt> structure, only
+one access out of sixteen will progress up the tree.
+For the internal <tt>rcu_node</tt> structures, the situation is even
+more extreme: Only one access out of sixty-four will progress up
+the tree.
+Because the vast majority of the CPUs do not progress up the tree,
+the lock contention remains roughly constant up the tree.
+No matter how many CPUs there are in the system, at most 64 quiescent-state
+reports per grace period will progress all the way to the root
+<tt>rcu_node</tt> structure, thus ensuring that the lock contention
+on that root <tt>rcu_node</tt> structure remains acceptably low.
+
+</p><p>In effect, the combining tree acts like a big shock absorber,
+keeping lock contention under control at all tree levels regardless
+of the level of loading on the system.
+
</p><p>The Linux kernel actually supports multiple flavors of RCU
running concurrently, so RCU builds separate data structures for each
flavor.
diff --git a/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx b/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx
index c08fd8e9574a..8e88e3e7e2ef 100644
--- a/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx
+++ b/Documentation/RCU/Design/Data-Structures/Data-Structures.htmlx
@@ -121,6 +121,29 @@ On the other hand, you can set <tt>CONFIG_RCU_FANOUT</tt> to be
as small as 2 if you wish, which would permit only 16 CPUs, which
is useful for testing.
+</p><p>This multi-level combining tree allows us to get most of the
+performance and scalability
+benefits of partitioning, even though RCU grace-period detection is
+inherently a global operation.
+The trick here is that only the last CPU to report a quiescent state
+into a given <tt>rcu_node</tt> structure need advance to the <tt>rcu_node</tt>
+structure at the next level up the tree.
+This means that at the leaf-level <tt>rcu_node</tt> structure, only
+one access out of sixteen will progress up the tree.
+For the internal <tt>rcu_node</tt> structures, the situation is even
+more extreme: Only one access out of sixty-four will progress up
+the tree.
+Because the vast majority of the CPUs do not progress up the tree,
+the lock contention remains roughly constant up the tree.
+No matter how many CPUs there are in the system, at most 64 quiescent-state
+reports per grace period will progress all the way to the root
+<tt>rcu_node</tt> structure, thus ensuring that the lock contention
+on that root <tt>rcu_node</tt> structure remains acceptably low.
+
+</p><p>In effect, the combining tree acts like a big shock absorber,
+keeping lock contention under control at all tree levels regardless
+of the level of loading on the system.
+
</p><p>The Linux kernel actually supports multiple flavors of RCU
running concurrently, so RCU builds separate data structures for each
flavor.
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
` (12 preceding siblings ...)
2016-02-24 5:00 ` [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention Paul E. McKenney
@ 2016-02-24 5:00 ` Paul E. McKenney
13 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 5:00 UTC (permalink / raw)
To: linux-kernel
Cc: mingo, jiangshanlai, dipankar, akpm, mathieu.desnoyers, josh,
tglx, peterz, rostedt, dhowells, edumazet, dvhart, fweisbec,
oleg, bobby.prani, SeongJae Park, Paul E. McKenney
From: SeongJae Park <sj38.park@gmail.com>
The compiler store-fusion example in memory-barriers.txt uses a C
comment to represent arbitrary code that does not update a given
variable. Unfortunately, someone could reasonably interpret the
comment as instead referring to the following line of code. This
commit therefore replaces the comment with a string that more
clearly represents the arbitrary code.
Signed-off-by: SeongJae Park <sj38.park@gmail.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/memory-barriers.txt | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index 8367d393cba2..3729cbe60e41 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -1550,7 +1550,7 @@ of optimizations:
the following:
a = 0;
- /* Code that does not store to variable a. */
+ ... Code that does not store to variable a ...
a = 0;
The compiler sees that the value of variable 'a' is already zero, so
@@ -1562,7 +1562,7 @@ of optimizations:
wrong guess:
WRITE_ONCE(a, 0);
- /* Code that does not store to variable a. */
+ ... Code that does not store to variable a ...
WRITE_ONCE(a, 0);
(*) The compiler is within its rights to reorder memory accesses unless
--
2.5.2
^ permalink raw reply related [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-24 5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
@ 2016-02-24 21:12 ` Mathieu Desnoyers
2016-02-24 21:40 ` Paul E. McKenney
0 siblings, 1 reply; 23+ messages in thread
From: Mathieu Desnoyers @ 2016-02-24 21:12 UTC (permalink / raw)
To: Paul E. McKenney
Cc: linux-kernel, Ingo Molnar, Lai Jiangshan, dipankar,
Andrew Morton, josh, Thomas Gleixner, Peter Zijlstra, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
----- On Feb 24, 2016, at 12:00 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:
> The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
> barrier() may be used to prevent compiler reordering when more than one
> leg of the control-dependent "if" statement start with identical stores.
> This is incorrect at high optimization levels. This commit therefore
> updates the summary to match the detailed description.
>
> Reported by: Jianyu Zhan <nasa4836@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> Documentation/memory-barriers.txt | 10 +++++++---
> 1 file changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/memory-barriers.txt
> b/Documentation/memory-barriers.txt
> index 904ee42d078e..e26058d3e253 100644
> --- a/Documentation/memory-barriers.txt
> +++ b/Documentation/memory-barriers.txt
> @@ -800,9 +800,13 @@ In summary:
> use smp_rmb(), smp_wmb(), or, in the case of prior stores and
> later loads, smp_mb().
>
> - (*) If both legs of the "if" statement begin with identical stores
> - to the same variable, a barrier() statement is required at the
> - beginning of each leg of the "if" statement.
> + (*) If both legs of the "if" statement begin with identical stores to
> + the same variable, then those stores must be ordered, either by
> + preceding both of them with smp_mb() or by using smp_store_release()
> + to carry out the stores. Please note that it is -not- sufficient
> + to use barrier() at beginning of each leg of the "if" statement,
> + as optimizing compilers do not necessarily respect barrier()
> + in this case.
Hrm, I really don't understand this one.
One caveat, as stated here, would be that optimizing compilers
can reorder instruction with respect to barrier() placed at the
beginning of if/else legs that start with identical stores.
It goes on stating that "smp_mb() or smp_store_release()" should
be used rather than barrier() in those cases.
I don't get how, from a compiler optimization perspective,
barrier() is any different from smp_mb().
#define barrier() __asm__ __volatile__("": : :"memory")
vs
#define mb() asm volatile("mfence":::"memory")
What the compiler would observe is a "memory" clobber in both
cases.
Now if the stated cause of this issue would have been
internal reordering of those identical stores within the
processor, I would understand that smp_mb() has an
effect which differs from the compiler barrier, but since
the paragraph begins by stating that this is purely for
compiler optimizations, I'm confused.
What am I missing there ?
Thanks,
Mathieu
>
> (*) Control dependencies require at least one run-time conditional
> between the prior load and the subsequent store, and this
> --
> 2.5.2
--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-24 21:12 ` Mathieu Desnoyers
@ 2016-02-24 21:40 ` Paul E. McKenney
2016-02-25 6:41 ` Jianyu Zhan
2016-02-25 8:21 ` Peter Zijlstra
0 siblings, 2 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-24 21:40 UTC (permalink / raw)
To: Mathieu Desnoyers
Cc: linux-kernel, Ingo Molnar, Lai Jiangshan, dipankar,
Andrew Morton, josh, Thomas Gleixner, Peter Zijlstra, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Wed, Feb 24, 2016 at 09:12:04PM +0000, Mathieu Desnoyers wrote:
> ----- On Feb 24, 2016, at 12:00 AM, Paul E. McKenney paulmck@linux.vnet.ibm.com wrote:
>
> > The summary of the "CONTROL DEPENDENCIES" section incorrectly states that
> > barrier() may be used to prevent compiler reordering when more than one
> > leg of the control-dependent "if" statement start with identical stores.
> > This is incorrect at high optimization levels. This commit therefore
> > updates the summary to match the detailed description.
> >
> > Reported by: Jianyu Zhan <nasa4836@gmail.com>
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > ---
> > Documentation/memory-barriers.txt | 10 +++++++---
> > 1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/Documentation/memory-barriers.txt
> > b/Documentation/memory-barriers.txt
> > index 904ee42d078e..e26058d3e253 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -800,9 +800,13 @@ In summary:
> > use smp_rmb(), smp_wmb(), or, in the case of prior stores and
> > later loads, smp_mb().
> >
> > - (*) If both legs of the "if" statement begin with identical stores
> > - to the same variable, a barrier() statement is required at the
> > - beginning of each leg of the "if" statement.
> > + (*) If both legs of the "if" statement begin with identical stores to
> > + the same variable, then those stores must be ordered, either by
> > + preceding both of them with smp_mb() or by using smp_store_release()
> > + to carry out the stores. Please note that it is -not- sufficient
> > + to use barrier() at beginning of each leg of the "if" statement,
> > + as optimizing compilers do not necessarily respect barrier()
> > + in this case.
>
> Hrm, I really don't understand this one.
>
> One caveat, as stated here, would be that optimizing compilers
> can reorder instruction with respect to barrier() placed at the
> beginning of if/else legs that start with identical stores.
>
> It goes on stating that "smp_mb() or smp_store_release()" should
> be used rather than barrier() in those cases.
>
> I don't get how, from a compiler optimization perspective,
> barrier() is any different from smp_mb().
>
> #define barrier() __asm__ __volatile__("": : :"memory")
>
> vs
>
> #define mb() asm volatile("mfence":::"memory")
>
> What the compiler would observe is a "memory" clobber in both
> cases.
>
> Now if the stated cause of this issue would have been
> internal reordering of those identical stores within the
> processor, I would understand that smp_mb() has an
> effect which differs from the compiler barrier, but since
> the paragraph begins by stating that this is purely for
> compiler optimizations, I'm confused.
>
> What am I missing there ?
>
> Thanks,
>
> Mathieu
>
>
> >
> > (*) Control dependencies require at least one run-time conditional
> > between the prior load and the subsequent store, and this
Let's take the example, replace barrier() with smp_mb(), and see what
happens:
q = READ_ONCE(a);
if (q) {
smp_mb();
WRITE_ONCE(b, p);
do_something();
} else {
smp_mb();
WRITE_ONCE(b, p);
do_something_else();
}
Given the same compiler transformation:
q = READ_ONCE(a);
smp_mb();
WRITE_ONCE(b, p); /* BUG: No ordering vs. load from a!!! */
if (q) {
/* WRITE_ONCE(b, p); -- moved up, BUG!!! */
do_something();
} else {
/* WRITE_ONCE(b, p); -- moved up, BUG!!! */
do_something_else();
}
So ordering between the read from "a" and the write to "b" is still
preserved. The reason this works is that the smp_mb() does all the
ordering, so the fact that the control dependency has been eliminated
is irrelevant.
Thanx, Paul
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-24 21:40 ` Paul E. McKenney
@ 2016-02-25 6:41 ` Jianyu Zhan
2016-02-25 14:08 ` Paul E. McKenney
2016-02-25 8:21 ` Peter Zijlstra
1 sibling, 1 reply; 23+ messages in thread
From: Jianyu Zhan @ 2016-02-25 6:41 UTC (permalink / raw)
To: Paul McKenney
Cc: Mathieu Desnoyers, LKML, Ingo Molnar, Lai Jiangshan, dipankar,
Andrew Morton, josh, Thomas Gleixner, Peter Zijlstra, rostedt,
David Howells, Eric Dumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Thu, Feb 25, 2016 at 5:40 AM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> So ordering between the read from "a" and the write to "b" is still
> preserved. The reason this works is that the smp_mb() does all the
> ordering, so the fact that the control dependency has been eliminated
> is irrelevant.
Thanks, Paul, nice clarification.
I thinks this example qualifies as an good example to demonstrate the subtle
scope of effect of these two level barriers.
A vivid example is always better for understanding instead of learning
"barrier() is compiler-level
barrier, and smp_*mb() is processor-level barrier" by rote ,
especially for the new comers to memory-barriers.txt. ;-)
Thanks,
Jianyu Zhan
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-24 21:40 ` Paul E. McKenney
2016-02-25 6:41 ` Jianyu Zhan
@ 2016-02-25 8:21 ` Peter Zijlstra
2016-02-25 14:07 ` Paul E. McKenney
1 sibling, 1 reply; 23+ messages in thread
From: Peter Zijlstra @ 2016-02-25 8:21 UTC (permalink / raw)
To: Paul E. McKenney
Cc: Mathieu Desnoyers, linux-kernel, Ingo Molnar, Lai Jiangshan,
dipankar, Andrew Morton, josh, Thomas Gleixner, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Wed, Feb 24, 2016 at 01:40:13PM -0800, Paul E. McKenney wrote:
> > > + (*) If both legs of the "if" statement begin with identical stores to
> > > + the same variable, then those stores must be ordered, either by
> > > + preceding both of them with smp_mb() or by using smp_store_release()
> > > + to carry out the stores. Please note that it is -not- sufficient
> > > + to use barrier() at beginning of each leg of the "if" statement,
> > > + as optimizing compilers do not necessarily respect barrier()
> > > + in this case.
> Let's take the example, replace barrier() with smp_mb(), and see what
> happens:
>
> q = READ_ONCE(a);
> if (q) {
> smp_mb();
> WRITE_ONCE(b, p);
> do_something();
> } else {
> smp_mb();
> WRITE_ONCE(b, p);
> do_something_else();
> }
Why would an optimizing compiler be allowed to lift _anything_ over a
barrier() ? Isn't that a bug?
I thought the whole point of barrier() was to tell the compiler to not
do such things.
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-25 8:21 ` Peter Zijlstra
@ 2016-02-25 14:07 ` Paul E. McKenney
2016-02-25 14:48 ` Peter Zijlstra
0 siblings, 1 reply; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-25 14:07 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mathieu Desnoyers, linux-kernel, Ingo Molnar, Lai Jiangshan,
dipankar, Andrew Morton, josh, Thomas Gleixner, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Thu, Feb 25, 2016 at 09:21:43AM +0100, Peter Zijlstra wrote:
> On Wed, Feb 24, 2016 at 01:40:13PM -0800, Paul E. McKenney wrote:
> > > > + (*) If both legs of the "if" statement begin with identical stores to
> > > > + the same variable, then those stores must be ordered, either by
> > > > + preceding both of them with smp_mb() or by using smp_store_release()
> > > > + to carry out the stores. Please note that it is -not- sufficient
> > > > + to use barrier() at beginning of each leg of the "if" statement,
> > > > + as optimizing compilers do not necessarily respect barrier()
> > > > + in this case.
>
> > Let's take the example, replace barrier() with smp_mb(), and see what
> > happens:
> >
> > q = READ_ONCE(a);
> > if (q) {
> > smp_mb();
> > WRITE_ONCE(b, p);
> > do_something();
> > } else {
> > smp_mb();
> > WRITE_ONCE(b, p);
> > do_something_else();
> > }
>
> Why would an optimizing compiler be allowed to lift _anything_ over a
> barrier() ? Isn't that a bug?
>
> I thought the whole point of barrier() was to tell the compiler to not
> do such things.
Still bad wording...
It hasn't actually moved anything over the barrier(). It has instead
moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if
(q)". Mathieu mentioned this over IRC yesterday, and I queue a change
so that the paragraph now reads as follows:
(*) If both legs of the "if" statement begin with identical stores to
the same variable, then those stores must be ordered, either by
preceding both of them with smp_mb() or by using smp_store_release()
to carry out the stores. Please note that it is -not- sufficient
to use barrier() at beginning of each leg of the "if" statement
because, as shown by the example above, optimizing compilers can
destroy the control dependency while respecting the letter of the
barrier() law.
Does hat help?
Thanx, Paul
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-25 6:41 ` Jianyu Zhan
@ 2016-02-25 14:08 ` Paul E. McKenney
0 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-25 14:08 UTC (permalink / raw)
To: Jianyu Zhan
Cc: Mathieu Desnoyers, LKML, Ingo Molnar, Lai Jiangshan, dipankar,
Andrew Morton, josh, Thomas Gleixner, Peter Zijlstra, rostedt,
David Howells, Eric Dumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Thu, Feb 25, 2016 at 02:41:07PM +0800, Jianyu Zhan wrote:
> On Thu, Feb 25, 2016 at 5:40 AM, Paul E. McKenney
> <paulmck@linux.vnet.ibm.com> wrote:
> > So ordering between the read from "a" and the write to "b" is still
> > preserved. The reason this works is that the smp_mb() does all the
> > ordering, so the fact that the control dependency has been eliminated
> > is irrelevant.
>
>
> Thanks, Paul, nice clarification.
>
> I thinks this example qualifies as an good example to demonstrate the subtle
> scope of effect of these two level barriers.
>
> A vivid example is always better for understanding instead of learning
> "barrier() is compiler-level
> barrier, and smp_*mb() is processor-level barrier" by rote ,
> especially for the new comers to memory-barriers.txt. ;-)
Glad you like it. ;-)
Thanx, Paul
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-25 14:07 ` Paul E. McKenney
@ 2016-02-25 14:48 ` Peter Zijlstra
2016-02-25 15:42 ` Paul E. McKenney
0 siblings, 1 reply; 23+ messages in thread
From: Peter Zijlstra @ 2016-02-25 14:48 UTC (permalink / raw)
To: Paul E. McKenney
Cc: Mathieu Desnoyers, linux-kernel, Ingo Molnar, Lai Jiangshan,
dipankar, Andrew Morton, josh, Thomas Gleixner, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Thu, Feb 25, 2016 at 06:07:03AM -0800, Paul E. McKenney wrote:
> Still bad wording...
>
> It hasn't actually moved anything over the barrier(). It has instead
> moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if
> (q)". Mathieu mentioned this over IRC yesterday, and I queue a change
> so that the paragraph now reads as follows:
>
> (*) If both legs of the "if" statement begin with identical stores to
> the same variable, then those stores must be ordered, either by
> preceding both of them with smp_mb() or by using smp_store_release()
> to carry out the stores. Please note that it is -not- sufficient
> to use barrier() at beginning of each leg of the "if" statement
> because, as shown by the example above, optimizing compilers can
> destroy the control dependency while respecting the letter of the
> barrier() law.
>
> Does hat help?
Maybe.. I still feel the compiler should not do this; but I'm having a
hard time explaining why.
^ permalink raw reply [flat|nested] 23+ messages in thread
* Re: [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores
2016-02-25 14:48 ` Peter Zijlstra
@ 2016-02-25 15:42 ` Paul E. McKenney
0 siblings, 0 replies; 23+ messages in thread
From: Paul E. McKenney @ 2016-02-25 15:42 UTC (permalink / raw)
To: Peter Zijlstra
Cc: Mathieu Desnoyers, linux-kernel, Ingo Molnar, Lai Jiangshan,
dipankar, Andrew Morton, josh, Thomas Gleixner, rostedt,
David Howells, edumazet, dvhart, fweisbec, Oleg Nesterov,
bobby prani
On Thu, Feb 25, 2016 at 03:48:38PM +0100, Peter Zijlstra wrote:
> On Thu, Feb 25, 2016 at 06:07:03AM -0800, Paul E. McKenney wrote:
> > Still bad wording...
> >
> > It hasn't actually moved anything over the barrier(). It has instead
> > moved both the barrier() and the WRITE_ONCE(b, p) to precede the "if
> > (q)". Mathieu mentioned this over IRC yesterday, and I queue a change
> > so that the paragraph now reads as follows:
> >
> > (*) If both legs of the "if" statement begin with identical stores to
> > the same variable, then those stores must be ordered, either by
> > preceding both of them with smp_mb() or by using smp_store_release()
> > to carry out the stores. Please note that it is -not- sufficient
> > to use barrier() at beginning of each leg of the "if" statement
> > because, as shown by the example above, optimizing compilers can
> > destroy the control dependency while respecting the letter of the
> > barrier() law.
> >
> > Does hat help?
>
> Maybe.. I still feel the compiler should not do this; but I'm having a
> hard time explaining why.
The compiler guys would probably argue that the variable "q" is in a
register, so that the "memory" constraint does not apply. Then the code
movement does not move any actual memory accesses across the "memory"
constraint, and hence they would argue them to be perfectly legal.
That said, I completely agree that this sort of code movement is at
the very least rather unfriendly to concurrent code. :-/
Thanx, Paul
^ permalink raw reply [flat|nested] 23+ messages in thread
end of thread, other threads:[~2016-02-25 15:42 UTC | newest]
Thread overview: 23+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2016-02-24 5:00 [PATCH tip/core/rcu 0/14] Documentation updates for 4.6 Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 01/14] documentation: Add real-time requirements from CPU-bound workloads Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 02/14] documentation: Fix control dependency and identical stores Paul E. McKenney
2016-02-24 21:12 ` Mathieu Desnoyers
2016-02-24 21:40 ` Paul E. McKenney
2016-02-25 6:41 ` Jianyu Zhan
2016-02-25 14:08 ` Paul E. McKenney
2016-02-25 8:21 ` Peter Zijlstra
2016-02-25 14:07 ` Paul E. McKenney
2016-02-25 14:48 ` Peter Zijlstra
2016-02-25 15:42 ` Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 03/14] documentation: Fix memory-barriers.txt section references Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 04/14] documentation: Add synchronize_rcu_mult() to the requirements Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 05/14] documentation: Remove obsolete reference to RCU-protected indexes Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 06/14] documentation: Subsequent writes ordered by rcu_dereference() Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 07/14] documentation: Distinguish between local and global transitivity Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 08/14] documentation: Add alternative release-acquire outcome Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 09/14] documentation: Add documentation for RCU's major data structures Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 10/14] documentation: Explain why rcu_read_lock() needs no barrier() Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 11/14] documentation: Transitivity is not cumulativity Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 12/14] documentation: Document illegality of call_rcu() from offline CPUs Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 13/14] documentation: Explain how RCU's combining tree fights contention Paul E. McKenney
2016-02-24 5:00 ` [PATCH tip/core/rcu 14/14] documentation: Clarify compiler store-fusion example Paul E. McKenney
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