sched: Make sure to not re-read variables after validation

We could re-read rq->rt_avg after we validated it was smaller than
total, invalidating the check and resulting in an unintended negative.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: David Rientjes <rientjes@google.com>
Link: http://lkml.kernel.org/r/1337688268.9698.29.camel@twins
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index f0380d4987b3a02ecaa9217a0de404c31064cea2..2b449a76207492013557431d6d554f33ae43f4a0 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3503,15 +3503,22 @@ unsigned long __weak arch_scale_smt_power(struct sched_domain *sd, int cpu)
 unsigned long scale_rt_power(int cpu)
 {
        struct rq *rq = cpu_rq(cpu);
-       u64 total, available;
+       u64 total, available, age_stamp, avg;
 
-       total = sched_avg_period() + (rq->clock - rq->age_stamp);
+       /*
+        * Since we're reading these variables without serialization make sure
+        * we read them once before doing sanity checks on them.
+        */
+       age_stamp = ACCESS_ONCE(rq->age_stamp);
+       avg = ACCESS_ONCE(rq->rt_avg);
+
+       total = sched_avg_period() + (rq->clock - age_stamp);
 
-       if (unlikely(total < rq->rt_avg)) {
+       if (unlikely(total < avg)) {
                /* Ensures that power won't end up being negative */
                available = 0;
        } else {
-               available = total - rq->rt_avg;
+               available = total - avg;
        }
 
        if (unlikely((s64)total < SCHED_POWER_SCALE))