* [PATCH tip/core/rcu 0/2] Documentation updates for v5.3
@ 2019-05-30 14:55 Paul E. McKenney
2019-05-30 14:55 ` [PATCH tip/core/rcu 1/2] doc/rcuref: Document real world examples in kernel Paul E. McKenney
2019-05-30 14:55 ` [PATCH tip/core/rcu 2/2] doc: Fixup definition of rcupdate.rcu_task_stall_timeout Paul E. McKenney
0 siblings, 2 replies; 3+ messages in thread
From: Paul E. McKenney @ 2019-05-30 14:55 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, mingo, jiangshanlai, dipankar, akpm,
mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel
Hello!
This series provides a couple of documentation updates:
1. Document real world rcuref examples from the Linux kernel,
courtesy of Joel Fernandes.
2. Fixup definition of rcupdate.rcu_task_stall_timeout, courtesy
of Zhenzhong Duan.
Thanx, Paul
------------------------------------------------------------------------
rcuref.txt | 21 ++++++++++++++++++++-
stallwarn.txt | 2 +-
2 files changed, 21 insertions(+), 2 deletions(-)
^ permalink raw reply [flat|nested] 3+ messages in thread
* [PATCH tip/core/rcu 1/2] doc/rcuref: Document real world examples in kernel
2019-05-30 14:55 [PATCH tip/core/rcu 0/2] Documentation updates for v5.3 Paul E. McKenney
@ 2019-05-30 14:55 ` Paul E. McKenney
2019-05-30 14:55 ` [PATCH tip/core/rcu 2/2] doc: Fixup definition of rcupdate.rcu_task_stall_timeout Paul E. McKenney
1 sibling, 0 replies; 3+ messages in thread
From: Paul E. McKenney @ 2019-05-30 14:55 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, mingo, jiangshanlai, dipankar, akpm,
mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, jannh, Paul E . McKenney
From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Document similar real world examples in the kernel corresponding to the
second and third code snippets. Also correct an issue in
release_referenced() in the code snippet example.
Cc: oleg@redhat.com
Cc: jannh@google.com
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
[ paulmck: Do a bit of wordsmithing. ]
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
Documentation/RCU/rcuref.txt | 21 ++++++++++++++++++++-
1 file changed, 20 insertions(+), 1 deletion(-)
diff --git a/Documentation/RCU/rcuref.txt b/Documentation/RCU/rcuref.txt
index 613033ff2b9b..5e6429d66c24 100644
--- a/Documentation/RCU/rcuref.txt
+++ b/Documentation/RCU/rcuref.txt
@@ -12,6 +12,7 @@ please read on.
Reference counting on elements of lists which are protected by traditional
reader/writer spinlocks or semaphores are straightforward:
+CODE LISTING A:
1. 2.
add() search_and_reference()
{ {
@@ -28,7 +29,8 @@ add() search_and_reference()
release_referenced() delete()
{ {
... write_lock(&list_lock);
- atomic_dec(&el->rc, relfunc) ...
+ if(atomic_dec_and_test(&el->rc)) ...
+ kfree(el);
... remove_element
} write_unlock(&list_lock);
...
@@ -44,6 +46,7 @@ search_and_reference() could potentially hold reference to an element which
has already been deleted from the list/array. Use atomic_inc_not_zero()
in this scenario as follows:
+CODE LISTING B:
1. 2.
add() search_and_reference()
{ {
@@ -79,6 +82,7 @@ search_and_reference() code path. In such cases, the
atomic_dec_and_test() may be moved from delete() to el_free()
as follows:
+CODE LISTING C:
1. 2.
add() search_and_reference()
{ {
@@ -114,6 +118,17 @@ element can therefore safely be freed. This in turn guarantees that if
any reader finds the element, that reader may safely acquire a reference
without checking the value of the reference counter.
+A clear advantage of the RCU-based pattern in listing C over the one
+in listing B is that any call to search_and_reference() that locates
+a given object will succeed in obtaining a reference to that object,
+even given a concurrent invocation of delete() for that same object.
+Similarly, a clear advantage of both listings B and C over listing A is
+that a call to delete() is not delayed even if there are an arbitrarily
+large number of calls to search_and_reference() searching for the same
+object that delete() was invoked on. Instead, all that is delayed is
+the eventual invocation of kfree(), which is usually not a problem on
+modern computer systems, even the small ones.
+
In cases where delete() can sleep, synchronize_rcu() can be called from
delete(), so that el_free() can be subsumed into delete as follows:
@@ -130,3 +145,7 @@ delete()
kfree(el);
...
}
+
+As additional examples in the kernel, the pattern in listing C is used by
+reference counting of struct pid, while the pattern in listing B is used by
+struct posix_acl.
--
2.17.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* [PATCH tip/core/rcu 2/2] doc: Fixup definition of rcupdate.rcu_task_stall_timeout
2019-05-30 14:55 [PATCH tip/core/rcu 0/2] Documentation updates for v5.3 Paul E. McKenney
2019-05-30 14:55 ` [PATCH tip/core/rcu 1/2] doc/rcuref: Document real world examples in kernel Paul E. McKenney
@ 2019-05-30 14:55 ` Paul E. McKenney
1 sibling, 0 replies; 3+ messages in thread
From: Paul E. McKenney @ 2019-05-30 14:55 UTC (permalink / raw)
To: rcu
Cc: linux-kernel, mingo, jiangshanlai, dipankar, akpm,
mathieu.desnoyers, josh, tglx, peterz, rostedt, dhowells,
edumazet, fweisbec, oleg, joel, Zhenzhong Duan,
Paul E . McKenney
From: Zhenzhong Duan <zhenzhong.duan@oracle.com>
A positive value of rcupdate.rcu_task_stall_timeout is an interval
in seconds rather than jiffies.
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@oracle.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
---
Documentation/RCU/stallwarn.txt | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 1ab70c37921f..13e88fc00f01 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -153,7 +153,7 @@ rcupdate.rcu_task_stall_timeout
This boot/sysfs parameter controls the RCU-tasks stall warning
interval. A value of zero or less suppresses RCU-tasks stall
warnings. A positive value sets the stall-warning interval
- in jiffies. An RCU-tasks stall warning starts with the line:
+ in seconds. An RCU-tasks stall warning starts with the line:
INFO: rcu_tasks detected stalls on tasks:
--
2.17.1
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