|author||Linus Torvalds <email@example.com>||2010-04-19 08:35:47 -0700|
|committer||Linus Torvalds <firstname.lastname@example.org>||2010-04-19 08:35:47 -0700|
Merge branch 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip
* 'core-fixes-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/linux-2.6-tip: rcu: Make RCU lockdep check the lockdep_recursion variable rcu: Update docs for rcu_access_pointer and rcu_dereference_protected rcu: Better explain the condition parameter of rcu_dereference_check() rcu: Add rcu_access_pointer and rcu_dereference_protected
Diffstat (limited to 'Documentation/RCU/NMI-RCU.txt')
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
@@ -34,7 +34,7 @@ NMI handler.
cpu = smp_processor_id();
- if (!rcu_dereference(nmi_callback)(regs, cpu))
+ if (!rcu_dereference_sched(nmi_callback)(regs, cpu))
@@ -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
+ More important, the rcu_dereference_sched() makes it
+ clear to someone reading the code that the pointer is
+ being protected by RCU-sched.