sched: Fix hrtimer_cancel()/rq->lock deadlock

__start_cfs_bandwidth calls hrtimer_cancel while holding rq->lock,
waiting for the hrtimer to finish. However, if sched_cfs_period_timer
runs for another loop iteration, the hrtimer can attempt to take
rq->lock, resulting in deadlock.

Fix this by ensuring that cfs_b->timer_active is cleared only if the
_latest_ call to do_sched_cfs_period_timer is returning as idle. Then
__start_cfs_bandwidth can just call hrtimer_try_to_cancel and wait for
that to succeed or timer_active == 1.

Signed-off-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: pjt@google.com
Link: http://lkml.kernel.org/r/20131016181622.22647.16643.stgit@sword-of-the-dawn.mtv.corp.google.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 897d977..f6308cb 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3225,6 +3225,13 @@ static int do_sched_cfs_period_timer(struct cfs_bandwidth *cfs_b, int overrun)
        if (idle)
                goto out_unlock;
 
+       /*
+        * if we have relooped after returning idle once, we need to update our
+        * status as actually running, so that other cpus doing
+        * __start_cfs_bandwidth will stop trying to cancel us.
+        */
+       cfs_b->timer_active = 1;
+
        __refill_cfs_bandwidth_runtime(cfs_b);
 
        if (!throttled) {
@@ -3493,11 +3500,11 @@ void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
         * (timer_active==0 becomes visible before the hrtimer call-back
         * terminates).  In either case we ensure that it's re-programmed
         */
-       while (unlikely(hrtimer_active(&cfs_b->period_timer))) {
+       while (unlikely(hrtimer_active(&cfs_b->period_timer)) &&
+              hrtimer_try_to_cancel(&cfs_b->period_timer) < 0) {
+               /* bounce the lock to allow do_sched_cfs_period_timer to run */
                raw_spin_unlock(&cfs_b->lock);
-               /* ensure cfs_b->lock is available while we wait */
-               hrtimer_cancel(&cfs_b->period_timer);
-
+               cpu_relax();
                raw_spin_lock(&cfs_b->lock);
                /* if someone else restarted the timer then we're done */
                if (cfs_b->timer_active)