linux-kernel - [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work

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Date:   Wed, 12 Apr 2017 09:55:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.org, jiangshanlai@...il.com, dipankar@...ibm.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 07/13] rcu: Add smp_mb__after_atomic() to sync_exp_work_done()

The sync_exp_work_done() function needs to fully order the
counter-check operation against anything happening after the
corresponding grace period.  This is a theoretical bug, as all current
architectures either provide full ordering for atomic operations
on the one hand or implement smp_mb__before_atomic() as smp_mb()
on the other.  However, a little future-proofing is a good thing,
especially given that smp_mb__before_atomic() is only required to
provide acquire semantics rather than full ordering.  This commit
therefore adds smp_mb__after_atomic() after the atomic_long_inc()
in sync_exp_work_done().

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
---
 kernel/rcu/tree_exp.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index a7b639ccd46e..e0cafa5f3269 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -247,6 +247,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
 		/* Ensure test happens before caller kfree(). */
 		smp_mb__before_atomic(); /* ^^^ */
 		atomic_long_inc(stat);
+		smp_mb__after_atomic(); /* ^^^ */
 		return true;
 	}
 	return false;
-- 
2.5.2