[PATCH] fix misplaced mb() in rcu_enter/exit_nohz()

[Paul E. McKenney]

Hello!

In the process of writing up the mechanical proof of correctness for the
dynticks/preemptable-RCU interface, I noticed misplaced memory barriers
in rcu_enter_nohz() and rcu_exit_nohz().  This patch puts them in the
right place and adds a comment.  The key thing to keep in mind is that
rcu_enter_nohz() is -exiting- the mode that can legally execute RCU
read-side critical sections.  The memory barrier must be between any
potential RCU read-side critical sections and the increment of the per-CPU
dynticks_progress_counter, and thus must come -before- this increment.
And vice versa for rcu_exit_nohz().

The locking in the scheduler is probably saving us for the moment.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---

 rcupreempt.h |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff -urpNa -X dontdiff linux-2.6.25-rc6/include/linux/rcupreempt.h linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h
--- linux-2.6.25-rc6/include/linux/rcupreempt.h	2008-03-16 17:45:16.000000000 -0700
+++ linux-2.6.25-rc6-rcu_nohz-fix/include/linux/rcupreempt.h	2008-03-16 17:59:24.000000000 -0700
@@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_
 
 static inline void rcu_enter_nohz(void)
 {
+	mb(); /* CPUs seeing ++ must see prior RCU read-side crit sects */
 	__get_cpu_var(dynticks_progress_counter)++;
 	WARN_ON(__get_cpu_var(dynticks_progress_counter) & 0x1);
-	mb();
 }
 
 static inline void rcu_exit_nohz(void)
 {
-	mb();
 	__get_cpu_var(dynticks_progress_counter)++;
+	mb(); /* CPUs seeing ++ must see subsequent RCU read-side crit sects */
 	WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
 }
 
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