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doc: Converted NMI-RCU.txt to NMI-RCU.rst.

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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>
This commit is contained in:
Madhuparna Bhowmik 2019-10-29 03:12:52 +05:30 committed by Paul E. McKenney
parent 9ffdd79824
commit 6705cae433
2 changed files with 29 additions and 25 deletions
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53
Documentation/RCU/NMI-RCU.txt → Documentation/RCU/NMI-RCU.rst
View File

@ -1,4 +1,7 @@
.. _NMI_rcu_doc:
Using RCU to Protect Dynamic NMI Handlers
=========================================
Although RCU is usually used to protect read-mostly data structures,
@ -9,7 +12,7 @@ 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.
brief explanation::
static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
{
@ -18,12 +21,12 @@ brief explanation.
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.
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.
NMI handler::
void do_nmi(struct pt_regs * regs, long error_code)
{
@ -53,11 +56,12 @@ 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?
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...
Back to the discussion of NMI and RCU::
void set_nmi_callback(nmi_callback_t callback)
{
@ -68,7 +72,7 @@ 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.
initialized values::
void unset_nmi_callback(void)
{
@ -82,7 +86,7 @@ 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:
follows::
unset_nmi_callback();
synchronize_rcu();
@ -98,24 +102,23 @@ 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
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?
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.
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.
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.
More important, the rcu_dereference_sched() makes it
clear to someone reading the code that the pointer is
being protected by RCU-sched.

1
Documentation/RCU/index.rst
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@ -10,6 +10,7 @@ RCU concepts
arrayRCU
rcu
listRCU
NMI-RCU
UP
Design/Memory-Ordering/Tree-RCU-Memory-Ordering