Commit 7c3f2ab7 authored Oct 15, 2013 by Peter Zijlstra Committed by Ingo Molnar Oct 16, 2013

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@infradead.org>
Cc: vincent.guittot@linaro.org
Cc: luca.abeni@unitn.it
Cc: bruce.ashfield@windriver.com
Cc: dhaval.giani@gmail.com
Cc: rostedt@goodmis.org
Cc: hgu1972@gmail.com
Cc: oleg@redhat.com
Cc: fweisbec@gmail.com
Cc: darren@dvhart.com
Cc: johan.eker@ericsson.com
Cc: p.faure@akatech.ch
Cc: paulmck@linux.vnet.ibm.com
Cc: raistlin@linux.it
Cc: claudio@evidence.eu.com
Cc: insop.song@gmail.com
Cc: michael@amarulasolutions.com
Cc: liming.wang@windriver.com
Cc: fchecconi@gmail.com
Cc: jkacur@redhat.com
Cc: tommaso.cucinotta@sssup.it
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: harald.gustafsson@ericsson.com
Cc: nicola.manica@disi.unitn.it
Cc: tglx@linutronix.de
Link: http://lkml.kernel.org/r/20131015103507.GF10651@twins.programming.kicks-ass.net


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent ed1b7732

kernel/sched/rt.c

+9 −1

Original line number	Diff line number	Diff line
		@@ -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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