From 09dc4ab03936df5c5aa711d27c81283c6d09f495 Mon Sep 17 00:00:00 2001 From: Roman Gushchin Date: Mon, 19 May 2014 15:10:09 +0400 Subject: [PATCH] sched/fair: Fix tg_set_cfs_bandwidth() deadlock on rq->lock 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 Reviewed-by: Ben Segall Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/87wqdi9g8e.wl\%klamm@yandex-team.ru Cc: pjt@google.com Cc: chris.j.arges@canonical.com Cc: gregkh@linuxfoundation.org Cc: Linus Torvalds Signed-off-by: Ingo Molnar --- 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 540542b93f9f..f3f48e7a2dda 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7751,8 +7751,7 @@ 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 b4768c069392..8cbe2d2c16de 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -3130,7 +3130,7 @@ 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) { @@ -3309,7 +3309,7 @@ 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); } @@ -3691,7 +3691,7 @@ 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 @@ -3706,7 +3706,7 @@ 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 456e492a3dca..369b4d663c42 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -278,7 +278,7 @@ 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); -- 2.20.1