linux-kernel - [tip:sched/core] sched/rt: Add missing rmb()

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Date:	Thu, 17 Oct 2013 09:49:20 -0700
From:	tip-bot for Peter Zijlstra <tipbot@...or.com>
To:	linux-tip-commits@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, hpa@...or.com, mingo@...nel.org,
	peterz@...radead.org, tglx@...utronix.de, juri.lelli@...il.com
Subject: [tip:sched/core] sched/rt: Add missing rmb()

Commit-ID:  7c3f2ab7b844f1a859afbc3d41925e8a0faba5fa
Gitweb:     http://git.kernel.org/tip/7c3f2ab7b844f1a859afbc3d41925e8a0faba5fa
Author:     Peter Zijlstra <peterz@...radead.org>
AuthorDate: Tue, 15 Oct 2013 12:35:07 +0200
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Wed, 16 Oct 2013 14:22:13 +0200

sched/rt: Add missing rmb()

While discussing the proposed SCHED_DEADLINE patches which in parts
mimic the existing FIFO code it was noticed that the wmb in
rt_set_overloaded() didn't have a matching barrier.

The only site using rt_overloaded() to test the rto_count is
pull_rt_task() and we should issue a matching rmb before then assuming
there's an rto_mask bit set.

Without that smp_rmb() in there we could actually miss seeing the
rto_mask bit.

Also, change to using smp_[wr]mb(), even though this is SMP only code;
memory barriers without smp_ always make me think they're against
hardware of some sort.

Signed-off-by: Peter Zijlstra <peterz@...radead.org>
Cc: vincent.guittot@...aro.org
Cc: luca.abeni@...tn.it
Cc: bruce.ashfield@...driver.com
Cc: dhaval.giani@...il.com
Cc: rostedt@...dmis.org
Cc: hgu1972@...il.com
Cc: oleg@...hat.com
Cc: fweisbec@...il.com
Cc: darren@...art.com
Cc: johan.eker@...csson.com
Cc: p.faure@...tech.ch
Cc: paulmck@...ux.vnet.ibm.com
Cc: raistlin@...ux.it
Cc: claudio@...dence.eu.com
Cc: insop.song@...il.com
Cc: michael@...rulasolutions.com
Cc: liming.wang@...driver.com
Cc: fchecconi@...il.com
Cc: jkacur@...hat.com
Cc: tommaso.cucinotta@...up.it
Cc: Juri Lelli <juri.lelli@...il.com>
Cc: harald.gustafsson@...csson.com
Cc: nicola.manica@...i.unitn.it
Cc: tglx@...utronix.de
Link: http://lkml.kernel.org/r/20131015103507.GF10651@twins.programming.kicks-ass.net
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/rt.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index e9304cd..a848f52 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -246,8 +246,10 @@ static inline void rt_set_overload(struct rq *rq)
 	 * if we should look at the mask. It would be a shame
 	 * if we looked at the mask, but the mask was not
 	 * updated yet.
+	 *
+	 * Matched by the barrier in pull_rt_task().
 	 */
-	wmb();
+	smp_wmb();
 	atomic_inc(&rq->rd->rto_count);
 }
 
@@ -1626,6 +1628,12 @@ static int pull_rt_task(struct rq *this_rq)
 	if (likely(!rt_overloaded(this_rq)))
 		return 0;
 
+	/*
+	 * Match the barrier from rt_set_overloaded; this guarantees that if we
+	 * see overloaded we must also see the rto_mask bit.
+	 */
+	smp_rmb();
+
 	for_each_cpu(cpu, this_rq->rd->rto_mask) {
 		if (this_cpu == cpu)
 			continue;
--
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