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>

diff --git a/Documentation/RCU/NMI-RCU.rst b/Documentation/RCU/NMI-RCU.rst
new file mode 100644 (file)
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 (file)
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 (file)
--- 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