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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Mar 17, 2008 at 02:09:06PM +1100, Nick Piggin wrote:
> On Monday 17 March 2008 12:08, Paul E. McKenney wrote:
> > 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@linux.vnet.ibm.com>
> > ---
> >
> >  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));
> >  }
> 
> Can you make these smp_mb() as well?

Can't see why not, now that you mention it.  Steve, anything Nick and
I are missing here?  (See updated patch below.)

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 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 22:53:07.000000000 -0700
@@ -87,15 +87,15 @@ DECLARE_PER_CPU(long, dynticks_progress_
 
 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));
 }