forked from luck/tmp_suning_uos_patched
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>
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Madhuparna Bhowmik
Paul E. McKenney
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9ffdd79824
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6705cae433
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@ -1,4 +1,7 @@
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.. _NMI_rcu_doc:
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Using RCU to Protect Dynamic NMI Handlers
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=========================================
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Although RCU is usually used to protect read-mostly data structures,
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@ -9,7 +12,7 @@ work in "arch/x86/oprofile/nmi_timer_int.c" and in
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"arch/x86/kernel/traps.c".
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The relevant pieces of code are listed below, each followed by a
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brief explanation.
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brief explanation::
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static int dummy_nmi_callback(struct pt_regs *regs, int cpu)
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{
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@ -18,12 +21,12 @@ brief explanation.
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The dummy_nmi_callback() function is a "dummy" NMI handler that does
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nothing, but returns zero, thus saying that it did nothing, allowing
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the NMI handler to take the default machine-specific action.
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the NMI handler to take the default machine-specific action::
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static nmi_callback_t nmi_callback = dummy_nmi_callback;
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This nmi_callback variable is a global function pointer to the current
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NMI handler.
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NMI handler::
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void do_nmi(struct pt_regs * regs, long error_code)
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{
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@ -53,11 +56,12 @@ anyway. However, in practice it is a good documentation aid, particularly
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for anyone attempting to do something similar on Alpha or on systems
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with aggressive optimizing compilers.
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Quick Quiz: Why might the rcu_dereference_sched() be necessary on Alpha,
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given that the code referenced by the pointer is read-only?
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Quick Quiz:
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Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only?
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:ref:`Answer to Quick Quiz <answer_quick_quiz_NMI>`
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Back to the discussion of NMI and RCU...
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Back to the discussion of NMI and RCU::
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void set_nmi_callback(nmi_callback_t callback)
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{
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@ -68,7 +72,7 @@ The set_nmi_callback() function registers an NMI handler. Note that any
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data that is to be used by the callback must be initialized up -before-
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the call to set_nmi_callback(). On architectures that do not order
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writes, the rcu_assign_pointer() ensures that the NMI handler sees the
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initialized values.
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initialized values::
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void unset_nmi_callback(void)
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{
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@ -82,7 +86,7 @@ up any data structures used by the old NMI handler until execution
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of it completes on all other CPUs.
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One way to accomplish this is via synchronize_rcu(), perhaps as
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follows:
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follows::
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unset_nmi_callback();
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synchronize_rcu();
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@ -98,24 +102,23 @@ to free up the handler's data as soon as synchronize_rcu() returns.
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Important note: for this to work, the architecture in question must
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invoke nmi_enter() and nmi_exit() on NMI entry and exit, respectively.
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.. _answer_quick_quiz_NMI:
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Answer to Quick Quiz
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Answer to Quick Quiz:
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Why might the rcu_dereference_sched() be necessary on Alpha, given that the code referenced by the pointer is read-only?
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Why might the rcu_dereference_sched() be necessary on Alpha, given
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that the code referenced by the pointer is read-only?
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The caller to set_nmi_callback() might well have
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initialized some data that is to be used by the new NMI
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handler. In this case, the rcu_dereference_sched() would
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be needed, because otherwise a CPU that received an NMI
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just after the new handler was set might see the pointer
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to the new NMI handler, but the old pre-initialized
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version of the handler's data.
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Answer: The caller to set_nmi_callback() might well have
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initialized some data that is to be used by the new NMI
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handler. In this case, the rcu_dereference_sched() would
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be needed, because otherwise a CPU that received an NMI
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just after the new handler was set might see the pointer
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to the new NMI handler, but the old pre-initialized
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version of the handler's data.
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This same sad story can happen on other CPUs when using
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a compiler with aggressive pointer-value speculation
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optimizations.
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This same sad story can happen on other CPUs when using
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a compiler with aggressive pointer-value speculation
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optimizations.
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More important, the rcu_dereference_sched() makes it
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clear to someone reading the code that the pointer is
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being protected by RCU-sched.
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More important, the rcu_dereference_sched() makes it
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clear to someone reading the code that the pointer is
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being protected by RCU-sched.
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@ -10,6 +10,7 @@ RCU concepts
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arrayRCU
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rcu
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listRCU
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NMI-RCU
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UP
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Design/Memory-Ordering/Tree-RCU-Memory-Ordering
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