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sched/fair: Fix tg_set_cfs_bandwidth() deadlock on rq->lock

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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 927b54fccb:
"__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@yandex-team.ru>
Reviewed-by: Ben Segall <bsegall@google.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
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 <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Roman Gushchin 2014-05-19 15:10:09 +04:00 committed by Ingo Molnar
parent 0f397f2c90
commit 09dc4ab039
3 changed files with 6 additions and 7 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

3
kernel/sched/core.c
View file

@ -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 */ /* restart the period timer (if active) to handle new period expiry */
if (runtime_enabled && cfs_b->timer_active) { if (runtime_enabled && cfs_b->timer_active) {
/* force a reprogram */ /* force a reprogram */
cfs_b->timer_active = 0; __start_cfs_bandwidth(cfs_b, true);
__start_cfs_bandwidth(cfs_b);
} }
raw_spin_unlock_irq(&cfs_b->lock); raw_spin_unlock_irq(&cfs_b->lock);

8
kernel/sched/fair.c
View file

@ -3130,7 +3130,7 @@ static int assign_cfs_rq_runtime(struct cfs_rq *cfs_rq)
*/ */
if (!cfs_b->timer_active) { if (!cfs_b->timer_active) {
__refill_cfs_bandwidth_runtime(cfs_b); __refill_cfs_bandwidth_runtime(cfs_b);
__start_cfs_bandwidth(cfs_b); __start_cfs_bandwidth(cfs_b, false);
} }
if (cfs_b->runtime > 0) { 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); raw_spin_lock(&cfs_b->lock);
list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq); list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
if (!cfs_b->timer_active) if (!cfs_b->timer_active)
__start_cfs_bandwidth(cfs_b); __start_cfs_bandwidth(cfs_b, false);
raw_spin_unlock(&cfs_b->lock); 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 */ /* 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 * 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(); cpu_relax();
raw_spin_lock(&cfs_b->lock); raw_spin_lock(&cfs_b->lock);
/* if someone else restarted the timer then we're done */ /* if someone else restarted the timer then we're done */
if (cfs_b->timer_active) if (!force && cfs_b->timer_active)
return; return;
} }

2
kernel/sched/sched.h
View file

@ -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 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 __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 unthrottle_cfs_rq(struct cfs_rq *cfs_rq);
extern void free_rt_sched_group(struct task_group *tg); extern void free_rt_sched_group(struct task_group *tg);