From: "tip-bot2 for Madhuparna Bhowmik" <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>,
Phong Tran <tranmanphong@gmail.com>,
"Paul E. McKenney" <paulmck@kernel.org>, x86 <x86@kernel.org>,
LKML <linux-kernel@vger.kernel.org>
Subject: [tip: core/rcu] doc: Converted NMI-RCU.txt to NMI-RCU.rst.
Date: Sat, 25 Jan 2020 10:42:50 -0000 [thread overview]
Message-ID: <157994897017.396.17671881349411288806.tip-bot2@tip-bot2> (raw)
The following commit has been merged into the core/rcu branch of tip:
Commit-ID: 6705cae433cffc37b183ded6ca9fe5c6d8ae8a9d
Gitweb: https://git.kernel.org/tip/6705cae433cffc37b183ded6ca9fe5c6d8ae8a9d
Author: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
AuthorDate: Tue, 29 Oct 2019 03:12:52 +05:30
Committer: Paul E. McKenney <paulmck@kernel.org>
CommitterDate: Tue, 10 Dec 2019 18:51:52 -08:00
doc: Converted NMI-RCU.txt to NMI-RCU.rst.
This patch converts NMI-RCU from txt to rst format.
Also adds NMI-RCU in the index.rst file.
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik04@gmail.com>
[ paulmck: Apply feedback from Phong Tran. ]
Tested-by: Phong Tran <tranmanphong@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
Documentation/RCU/NMI-RCU.rst | 124 +++++++++++++++++++++++++++++++++-
Documentation/RCU/NMI-RCU.txt | 121 +--------------------------------
Documentation/RCU/index.rst | 1 +-
3 files changed, 125 insertions(+), 121 deletions(-)
create mode 100644 Documentation/RCU/NMI-RCU.rst
delete mode 100644 Documentation/RCU/NMI-RCU.txt
diff --git a/Documentation/RCU/NMI-RCU.rst b/Documentation/RCU/NMI-RCU.rst
new file mode 100644
index 0000000..1809583
--- /dev/null
+++ b/Documentation/RCU/NMI-RCU.rst
@@ -0,0 +1,124 @@
+.. _NMI_rcu_doc:
+
+Using RCU to Protect Dynamic NMI Handlers
+=========================================
+
+
+Although RCU is usually used to protect read-mostly data structures,
+it is possible to use RCU to provide dynamic non-maskable interrupt
+handlers, as well as dynamic irq handlers. This document describes
+how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer
+work in "arch/x86/oprofile/nmi_timer_int.c" and in
+"arch/x86/kernel/traps.c".
+
+The relevant pieces of code are listed below, each followed by a
+brief explanation::
+
+ static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
+ {
+ return 0;
+ }
+
+The dummy_nmi_callback() function is a "dummy" NMI handler that does
+nothing, but returns zero, thus saying that it did nothing, allowing
+the NMI handler to take the default machine-specific action::
+
+ static nmi_callback_t nmi_callback = dummy_nmi_callback;
+
+This nmi_callback variable is a global function pointer to the current
+NMI handler::
+
+ void do_nmi(struct pt_regs * regs, long error_code)
+ {
+ int cpu;
+
+ nmi_enter();
+
+ cpu = smp_processor_id();
+ ++nmi_count(cpu);
+
+ if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
+ default_do_nmi(regs);
+
+ nmi_exit();
+ }
+
+The do_nmi() function processes each NMI. It first disables preemption
+in the same way that a hardware irq would, then increments the per-CPU
+count of NMIs. It then invokes the NMI handler stored in the nmi_callback
+function pointer. If this handler returns zero, do_nmi() invokes the
+default_do_nmi() function to handle a machine-specific NMI. Finally,
+preemption is restored.
+
+In theory, rcu_dereference_sched() is not needed, since this code runs
+only on i386, which in theory does not need rcu_dereference_sched()
+anyway. However, in practice it is a good documentation aid, particularly
+for anyone attempting to do something similar on Alpha or on systems
+with aggressive optimizing compilers.
+
+Quick Quiz:
+ Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only?
+
+:ref:`Answer to Quick Quiz <answer_quick_quiz_NMI>`
+
+Back to the discussion of NMI and RCU::
+
+ void set_nmi_callback(nmi_callback_t callback)
+ {
+ rcu_assign_pointer(nmi_callback, callback);
+ }
+
+The set_nmi_callback() function registers an NMI handler. Note that any
+data that is to be used by the callback must be initialized up -before-
+the call to set_nmi_callback(). On architectures that do not order
+writes, the rcu_assign_pointer() ensures that the NMI handler sees the
+initialized values::
+
+ void unset_nmi_callback(void)
+ {
+ rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
+ }
+
+This function unregisters an NMI handler, restoring the original
+dummy_nmi_handler(). However, there may well be an NMI handler
+currently executing on some other CPU. We therefore cannot free
+up any data structures used by the old NMI handler until execution
+of it completes on all other CPUs.
+
+One way to accomplish this is via synchronize_rcu(), perhaps as
+follows::
+
+ unset_nmi_callback();
+ synchronize_rcu();
+ kfree(my_nmi_data);
+
+This works because (as of v4.20) synchronize_rcu() blocks until all
+CPUs complete any preemption-disabled segments of code that they were
+executing.
+Since NMI handlers disable preemption, synchronize_rcu() is guaranteed
+not to return until all ongoing NMI handlers exit. It is therefore safe
+to free up the handler's data as soon as synchronize_rcu() returns.
+
+Important note: for this to work, the architecture in question must
+invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.
+
+.. _answer_quick_quiz_NMI:
+
+Answer to Quick Quiz:
+ Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only?
+
+ The caller to set_nmi_callback() might well have
+ initialized some data that is to be used by the new NMI
+ handler. In this case, the rcu_dereference_sched() would
+ be needed, because otherwise a CPU that received an NMI
+ just after the new handler was set might see the pointer
+ to the new NMI handler, but the old pre-initialized
+ version of the handler's data.
+
+ This same sad story can happen on other CPUs when using
+ a compiler with aggressive pointer-value speculation
+ optimizations.
+
+ More important, the rcu_dereference_sched() makes it
+ clear to someone reading the code that the pointer is
+ being protected by RCU-sched.
diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt
deleted file mode 100644
index 881353f..0000000
--- a/Documentation/RCU/NMI-RCU.txt
+++ /dev/null
@@ -1,121 +0,0 @@
-Using RCU to Protect Dynamic NMI Handlers
-
-
-Although RCU is usually used to protect read-mostly data structures,
-it is possible to use RCU to provide dynamic non-maskable interrupt
-handlers, as well as dynamic irq handlers. This document describes
-how to do this, drawing loosely from Zwane Mwaikambo's NMI-timer
-work in "arch/x86/oprofile/nmi_timer_int.c" and in
-"arch/x86/kernel/traps.c".
-
-The relevant pieces of code are listed below, each followed by a
-brief explanation.
-
- static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
- {
- return 0;
- }
-
-The dummy_nmi_callback() function is a "dummy" NMI handler that does
-nothing, but returns zero, thus saying that it did nothing, allowing
-the NMI handler to take the default machine-specific action.
-
- static nmi_callback_t nmi_callback = dummy_nmi_callback;
-
-This nmi_callback variable is a global function pointer to the current
-NMI handler.
-
- void do_nmi(struct pt_regs * regs, long error_code)
- {
- int cpu;
-
- nmi_enter();
-
- cpu = smp_processor_id();
- ++nmi_count(cpu);
-
- if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
- default_do_nmi(regs);
-
- nmi_exit();
- }
-
-The do_nmi() function processes each NMI. It first disables preemption
-in the same way that a hardware irq would, then increments the per-CPU
-count of NMIs. It then invokes the NMI handler stored in the nmi_callback
-function pointer. If this handler returns zero, do_nmi() invokes the
-default_do_nmi() function to handle a machine-specific NMI. Finally,
-preemption is restored.
-
-In theory, rcu_dereference_sched() is not needed, since this code runs
-only on i386, which in theory does not need rcu_dereference_sched()
-anyway. However, in practice it is a good documentation aid, particularly
-for anyone attempting to do something similar on Alpha or on systems
-with aggressive optimizing compilers.
-
-Quick Quiz: Why might the rcu_dereference_sched() be necessary on Alpha,
- given that the code referenced by the pointer is read-only?
-
-
-Back to the discussion of NMI and RCU...
-
- void set_nmi_callback(nmi_callback_t callback)
- {
- rcu_assign_pointer(nmi_callback, callback);
- }
-
-The set_nmi_callback() function registers an NMI handler. Note that any
-data that is to be used by the callback must be initialized up -before-
-the call to set_nmi_callback(). On architectures that do not order
-writes, the rcu_assign_pointer() ensures that the NMI handler sees the
-initialized values.
-
- void unset_nmi_callback(void)
- {
- rcu_assign_pointer(nmi_callback, dummy_nmi_callback);
- }
-
-This function unregisters an NMI handler, restoring the original
-dummy_nmi_handler(). However, there may well be an NMI handler
-currently executing on some other CPU. We therefore cannot free
-up any data structures used by the old NMI handler until execution
-of it completes on all other CPUs.
-
-One way to accomplish this is via synchronize_rcu(), perhaps as
-follows:
-
- unset_nmi_callback();
- synchronize_rcu();
- kfree(my_nmi_data);
-
-This works because (as of v4.20) synchronize_rcu() blocks until all
-CPUs complete any preemption-disabled segments of code that they were
-executing.
-Since NMI handlers disable preemption, synchronize_rcu() is guaranteed
-not to return until all ongoing NMI handlers exit. It is therefore safe
-to free up the handler's data as soon as synchronize_rcu() returns.
-
-Important note: for this to work, the architecture in question must
-invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.
-
-
-Answer to Quick Quiz
-
- Why might the rcu_dereference_sched() be necessary on Alpha, given
- that the code referenced by the pointer is read-only?
-
- Answer: The caller to set_nmi_callback() might well have
- initialized some data that is to be used by the new NMI
- handler. In this case, the rcu_dereference_sched() would
- be needed, because otherwise a CPU that received an NMI
- just after the new handler was set might see the pointer
- to the new NMI handler, but the old pre-initialized
- version of the handler's data.
-
- This same sad story can happen on other CPUs when using
- a compiler with aggressive pointer-value speculation
- optimizations.
-
- More important, the rcu_dereference_sched() makes it
- clear to someone reading the code that the pointer is
- being protected by RCU-sched.
diff --git a/Documentation/RCU/index.rst b/Documentation/RCU/index.rst
index 8d20d44..627128c 100644
--- a/Documentation/RCU/index.rst
+++ b/Documentation/RCU/index.rst
@@ -10,6 +10,7 @@ RCU concepts
arrayRCU
rcu
listRCU
+ NMI-RCU
UP
Design/Memory-Ordering/Tree-RCU-Memory-Ordering
reply other threads:[~2020-01-25 10:45 UTC|newest]
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