[PATCH] NMI: Update NMI users of RCU to use new API

Uses of RCU for dynamically changeable NMI handlers need to use the new
rcu_dereference() and rcu_assign_pointer() facilities.  This change makes
it clear that these uses are safe from a memory-barrier viewpoint, but the
main purpose is to document exactly what operations are being protected by
RCU.  This has been tested on x86 and x86-64, which are the only
architectures affected by this change.

Signed-off-by: <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt
new file mode 100644 (file)
index 0000000..d0634a5
--- /dev/null
+++ b/Documentation/RCU/NMI-RCU.txt
@@ -0,0 +1,112 @@
+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/i386/oprofile/nmi_timer_int.c" and in
+"arch/i386/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.
+
+       fastcall void do_nmi(struct pt_regs * regs, long error_code)
+       {
+               int cpu;
+
+               nmi_enter();
+
+               cpu = smp_processor_id();
+               ++nmi_count(cpu);
+
+               if (!rcu_dereference(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.
+
+Strictly speaking, rcu_dereference() is not needed, since this code runs
+only on i386, which does not need rcu_dereference() anyway.  However,
+it is a good documentation aid, particularly for anyone attempting to
+do something similar on Alpha.
+
+Quick Quiz:  Why might the rcu_dereference() 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_sched(), perhaps as
+follows:
+
+       unset_nmi_callback();
+       synchronize_sched();
+       kfree(my_nmi_data);
+
+This works because synchronize_sched() blocks until all CPUs complete
+any preemption-disabled segments of code that they were executing.
+Since NMI handlers disable preemption, synchronize_sched() 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_sched() returns.
+
+
+Answer to Quick Quiz
+
+       Why might the rcu_dereference() 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()
+               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.
+
+               More important, the rcu_dereference() makes it clear
+               to someone reading the code that the pointer is being
+               protected by RCU.

diff --git a/arch/i386/kernel/traps.c b/arch/i386/kernel/traps.c
index 54629bb..029bf94 100644 (file)
--- a/arch/i386/kernel/traps.c
+++ b/arch/i386/kernel/traps.c
@@ -657,7 +657,7 @@ fastcall void do_nmi(struct pt_regs * regs, long error_code)
 
        ++nmi_count(cpu);
 
-       if (!nmi_callback(regs, cpu))
+       if (!rcu_dereference(nmi_callback)(regs, cpu))
                default_do_nmi(regs);
 
        nmi_exit();
@@ -665,7 +665,7 @@ fastcall void do_nmi(struct pt_regs * regs, long error_code)
 
 void set_nmi_callback(nmi_callback_t callback)
 {
-       nmi_callback = callback;
+       rcu_assign_pointer(nmi_callback, callback);
 }
 EXPORT_SYMBOL_GPL(set_nmi_callback);
 

diff --git a/arch/x86_64/kernel/nmi.c b/arch/x86_64/kernel/nmi.c
index 84cae81..caf1649 100644 (file)
--- a/arch/x86_64/kernel/nmi.c
+++ b/arch/x86_64/kernel/nmi.c
@@ -524,14 +524,14 @@ asmlinkage void do_nmi(struct pt_regs * regs, long error_code)
 
        nmi_enter();
        add_pda(__nmi_count,1);
-       if (!nmi_callback(regs, cpu))
+       if (!rcu_dereference(nmi_callback)(regs, cpu))
                default_do_nmi(regs);
        nmi_exit();
 }
 
 void set_nmi_callback(nmi_callback_t callback)
 {
-       nmi_callback = callback;
+       rcu_assign_pointer(nmi_callback, callback);
 }
 
 void unset_nmi_callback(void)