sched/fair: Fix unlocked reads of some cfs_b->quota/period

sched_cfs_period_timer() reads cfs_b->period without locks before calling
do_sched_cfs_period_timer(), and similarly unthrottle_offline_cfs_rqs()
would read cfs_b->period without the right lock. Thus a simultaneous
change of bandwidth could cause corruption on any platform where ktime_t
or u64 writes/reads are not atomic.

Extend cfs_b->lock from do_sched_cfs_period_timer() to include the read of
cfs_b->period to solve that issue; unthrottle_offline_cfs_rqs() can just
use 1 rather than the exact quota, much like distribute_cfs_runtime()
does.

There is also an unlocked read of cfs_b->runtime_expires, but a race
there would only delay runtime expiry by a tick. Still, the comparison
should just be != anyway, which clarifies even that problem.

Signed-off-by: Ben Segall <bsegall@google.com>
Tested-by: Roman Gushchin <klamm@yandex-team.ru>
[peterz: Fix compile warn]
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20140519224945.20303.93530.stgit@sword-of-the-dawn.mtv.corp.google.com
Cc: pjt@google.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 21 insertions, 19 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c9617b73bcc0..b71d8c39f1fd 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3224,10 +3224,12 @@ static void expire_cfs_rq_runtime(struct cfs_rq *cfs_rq)
          * has not truly expired.
          *
          * Fortunately we can check determine whether this the case by checking
-         * whether the global deadline has advanced.
+         * whether the global deadline has advanced. It is valid to compare
+         * cfs_b->runtime_expires without any locks since we only care about
+         * exact equality, so a partial write will still work.
          */
 
-        if ((s64)(cfs_rq->runtime_expires - cfs_b->runtime_expires) >= 0) {
+        if (cfs_rq->runtime_expires != cfs_b->runtime_expires) {
                 /* extend local deadline, drift is bounded above by 2 ticks */
                 cfs_rq->runtime_expires += TICK_NSEC;
         } else {
@@ -3456,21 +3458,21 @@ next:
 static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
 {
         u64 runtime, runtime_expires;
-        int idle = 1, throttled;
+        int throttled;
 
-        raw_spin_lock(&cfs_b->lock);
         /* no need to continue the timer with no bandwidth constraint */
         if (cfs_b->quota == RUNTIME_INF)
-                goto out_unlock;
+                goto out_deactivate;
 
         throttled = !list_empty(&cfs_b->throttled_cfs_rq);
-        /* idle depends on !throttled (for the case of a large deficit) */
-        idle = cfs_b->idle && !throttled;
         cfs_b->nr_periods += overrun;
 
-        /* if we're going inactive then everything else can be deferred */
-        if (idle)
-                goto out_unlock;
+        /*
+         * idle depends on !throttled (for the case of a large deficit), and if
+         * we're going inactive then everything else can be deferred
+         */
+        if (cfs_b->idle && !throttled)
+                goto out_deactivate;
 
         /*
          * if we have relooped after returning idle once, we need to update our
@@ -3484,7 +3486,7 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
         if (!throttled) {
                 /* mark as potentially idle for the upcoming period */
                 cfs_b->idle = 1;
-                goto out_unlock;
+                return 0;
         }
 
         /* account preceding periods in which throttling occurred */
@@ -3524,12 +3526,12 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
          * timer to remain active while there are any throttled entities.)
          */
         cfs_b->idle = 0;
-out_unlock:
-        if (idle)
-                cfs_b->timer_active = 0;
-        raw_spin_unlock(&cfs_b->lock);
 
-        return idle;
+        return 0;
+
+out_deactivate:
+        cfs_b->timer_active = 0;
+        return 1;
 }
 
 /* a cfs_rq won't donate quota below this amount */
@@ -3706,6 +3708,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
         int overrun;
         int idle = 0;
 
+        raw_spin_lock(&cfs_b->lock);
         for (;;) {
                 now = hrtimer_cb_get_time(timer);
                 overrun = hrtimer_forward(timer, now, cfs_b->period);
@@ -3715,6 +3718,7 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
 
                 idle = do_sched_cfs_period_timer(cfs_b, overrun);
         }
+        raw_spin_unlock(&cfs_b->lock);
 
         return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
 }
@@ -3774,8 +3778,6 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
         struct cfs_rq *cfs_rq;
 
         for_each_leaf_cfs_rq(rq, cfs_rq) {
-                struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
-
                 if (!cfs_rq->runtime_enabled)
                         continue;
 
@@ -3783,7 +3785,7 @@ static void __maybe_unused unthrottle_offline_cfs_rqs(struct rq *rq)
                  * clock_task is not advancing so we just need to make sure
                  * there's some valid quota amount
                  */
-                cfs_rq->runtime_remaining = cfs_b->quota;
+                cfs_rq->runtime_remaining = 1;
                 if (cfs_rq_throttled(cfs_rq))
                         unthrottle_cfs_rq(cfs_rq);
         }