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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2008-03-19 17:00:57 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2008-03-19 18:53:36 -0700
commitae66be9b71b12f16b84129860d06bbfe37fbec51 (patch)
tree5bf1d1722f0d83bb4c30621b031437a141a15af1 /include/linux
parentb691da35ba94d5fa0a32589d99b6546be2bf4cfa (diff)
rcu: fix misplaced mb() in rcu_enter/exit_nohz()
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. Also, switch to smp_mb() - we don't need a barrier for uniprocessor kernels. Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Acked-by: Steven Rostedt <srostedt@redhat.com> Cc: Nick Piggin <nickpiggin@yahoo.com.au> Cc: Oleg Nesterov <oleg@tv-sign.ru> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux')
-rw-r--r--include/linux/rcupreempt.h4
1 files changed, 2 insertions, 2 deletions
diff --git a/include/linux/rcupreempt.h b/include/linux/rcupreempt.h
index 01152ed532c..d038aa6e5ee 100644
--- a/include/linux/rcupreempt.h
+++ b/include/linux/rcupreempt.h
@@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_counter);
static inline void rcu_enter_nohz(void)
{
+ smp_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)++;
+ smp_mb(); /* CPUs seeing ++ must see later RCU read-side crit sects */
WARN_ON(!(__get_cpu_var(dynticks_progress_counter) & 0x1));
}