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>

diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index e9304cdc26feafa244a4ce61f4aa59b58509b55c..a848f526b941a1cbda606bd954b927d7e7a8eb76 100644 (file)
--- 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;