Re: [PATCH tip/core/rcu 6/9] rcu: Upgrade sync_exp_work_done() to smp_mb()

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

On Thu, Jun 06, 2019 at 09:48:49AM +0200, Peter Zijlstra wrote:
> On Thu, May 30, 2019 at 08:00:12AM -0700, Paul E. McKenney wrote:
> > The sync_exp_work_done() function uses smp_mb__before_atomic(), but
> > there is no obvious atomic in the ensuing code.  The ordering is
> > absolutely required for grace periods to work correctly, so this
> > commit upgrades the smp_mb__before_atomic() to smp_mb().
> > 
> 
> Did this commit want a Fixes: line? Such that robots can find the right
> kernels to backport this to?

Indeed it does, and thanks to Andrea for finding the commit.

							Thanx, Paul

------------------------------------------------------------------------

commit 96050c68be33edef18800ad6748f61f81db81a20
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date:   Sat Apr 20 01:40:54 2019 -0700

    rcu: Upgrade sync_exp_work_done() to smp_mb()
    
    The sync_exp_work_done() function uses smp_mb__before_atomic(), but
    there is no obvious atomic in the ensuing code.  The ordering is
    absolutely required for grace periods to work correctly, so this
    commit upgrades the smp_mb__before_atomic() to smp_mb().
    
    Fixes: 6fba2b3767ea ("rcu: Remove deprecated RCU debugfs tracing code")
    Reported-by: Andrea Parri <andrea.parri@amarulasolutions.com>
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 9c990df880d1..d969650a72c6 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -259,8 +259,7 @@ static bool sync_exp_work_done(unsigned long s)
 {
 	if (rcu_exp_gp_seq_done(s)) {
 		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
-		/* Ensure test happens before caller kfree(). */
-		smp_mb__before_atomic(); /* ^^^ */
+		smp_mb(); /* Ensure test happens before caller kfree(). */
 		return true;
 	}
 	return false;