1 files changed, 22 insertions, 17 deletions

diff --git a/Documentation/RCU/NMI-RCU.txt b/Documentation/RCU/NMI-RCU.txt
index a6d32e65d222..a8536cb88091 100644
--- a/Documentation/RCU/NMI-RCU.txt
+++ b/Documentation/RCU/NMI-RCU.txt
@@ -34,7 +34,7 @@ NMI handler.
                 cpu = smp_processor_id();
                 ++nmi_count(cpu);
 
-                if (!rcu_dereference(nmi_callback)(regs, cpu))
+                if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
                         default_do_nmi(regs);
 
                 nmi_exit();
@@ -47,12 +47,13 @@ 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.
+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() be necessary on Alpha,
+Quick Quiz:  Why might the rcu_dereference_sched() be necessary on Alpha,
              given that the code referenced by the pointer is read-only?
 
 
@@ -99,17 +100,21 @@ invoke irq_enter() and irq_exit() on NMI entry and exit, respectively.
 
 Answer to Quick Quiz
 
-        Why might the rcu_dereference() be necessary on Alpha, given
+        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()
-                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.
+                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.