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Commit 927b54fc authored by Ben Segall's avatar Ben Segall Committed by Ingo Molnar
Browse files

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: default avatarBen Segall <bsegall@google.com>
Signed-off-by: default avatarPeter 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: default avatarIngo Molnar <mingo@kernel.org>
parent db06e78c

kernel/sched/fair.c

+11 −4
Original line number Diff line number Diff line
@@ -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)