linux-kernel - Re: [PATCH] sched: tg_set_cfs_bandwidth() causes rq->lock deadlock

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Date:	Mon, 19 May 2014 15:10:09 +0400
From:	Roman Gushchin <klamm@...dex-team.ru>
To:	bsegall@...gle.com
Cc:	Roman Gushchin <klamm@...dex-team.ru>,
	linux-kernel@...r.kernel.org, peterz@...radead.org, pjt@...gle.com,
	chris.j.arges@...onical.com, gregkh@...uxfoundation.org
Subject: Re: [PATCH] sched: tg_set_cfs_bandwidth() causes rq->lock deadlock

tg_set_cfs_bandwidth() sets cfs_b->timer_active to 0 to
force the period timer restart. It's not safe, because
can lead to deadlock, described in commit 927b54fccbf0:
"__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."
Three CPUs must be involved:
CPU0               CPU1                         CPU2
take rq->lock      period timer fired
...                take cfs_b lock
...                ...                          tg_set_cfs_bandwidth()
throttle_cfs_rq()  release cfs_b lock           take cfs_b lock
...                distribute_cfs_runtime()     timer_active = 0
take cfs_b->lock   wait for rq->lock            ...
__start_cfs_bandwidth()
{wait for timer callback
 break if timer_active == 1}

So, CPU0 and CPU1 are deadlocked.

Instead of resetting cfs_b->timer_active, tg_set_cfs_bandwidth can
wait for period timer callbacks (ignoring cfs_b->timer_active) and
restart the timer explicitly.

Signed-off-by: Roman Gushchin <klamm@...dex-team.ru>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: pjt@...gle.com
Cc: Chris J Arges <chris.j.arges@...onical.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/sched/core.c  |    3 +--
 kernel/sched/fair.c  |    8 ++++----
 kernel/sched/sched.h |    2 +-
 3 files changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b4308d7..e122774 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7826,8 +7826,7 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
 	/* restart the period timer (if active) to handle new period expiry */
 	if (runtime_enabled && cfs_b->timer_active) {
 		/* force a reprogram */
-		cfs_b->timer_active = 0;
-		__start_cfs_bandwidth(cfs_b);
+		__start_cfs_bandwidth(cfs_b, true);
 	}
 	raw_spin_unlock_irq(&cfs_b->lock);
 
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c7ab8ea..54939f8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2117,7 +2117,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 		 */
 		if (!cfs_b->timer_active) {
 			__refill_cfs_bandwidth_runtime(cfs_b);
-			__start_cfs_bandwidth(cfs_b);
+			__start_cfs_bandwidth(cfs_b, false);
 		}
 
 		if (cfs_b->runtime > 0) {
@@ -2298,7 +2298,7 @@ static void throttle_cfs_rq(struct cfs_rq *cfs_rq)
 	raw_spin_lock(&cfs_b->lock);
 	list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
 	if (!cfs_b->timer_active)
-		__start_cfs_bandwidth(cfs_b);
+		__start_cfs_bandwidth(cfs_b, false);
 	raw_spin_unlock(&cfs_b->lock);
 }
 
@@ -2681,7 +2681,7 @@ static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq)
 }
 
 /* requires cfs_b->lock, may release to reprogram timer */
-void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
+void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b, bool force)
 {
 	/*
 	 * The timer may be active because we're trying to set a new bandwidth
@@ -2696,7 +2696,7 @@ void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
 		cpu_relax();
 		raw_spin_lock(&cfs_b->lock);
 		/* if someone else restarted the timer then we're done */
-		if (cfs_b->timer_active)
+		if (!force && cfs_b->timer_active)
 			return;
 	}
 
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index dfa31d5..bc150a6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -209,7 +209,7 @@ extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
 extern int sched_group_set_shares(struct task_group *tg, unsigned long shares);
 
 extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
-extern void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
+extern void __start_cfs_bandwidth(struct cfs_bandwidth *cfs_b, bool force);
 extern void unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
 
 extern void free_rt_sched_group(struct task_group *tg);
-- 
1.7.9.5

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