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 <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>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 540542b93f9f9820fabb7c353907f391498bd4a2..f3f48e7a2ddae9cb9876dede2b1004bb2eaf19d2 100644 (file)
--- 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 b4768c0693927b1ce801502f79aa6e7fe9da1f80..8cbe2d2c16de7ba52a8ea2f3481dd9d59f5846f4 100644 (file)
--- 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 456e492a3dca37c13d7cb7b57a51965bfa18d6b3..369b4d663c42aea34ddba165332c30e9da2bc97f 100644 (file)
--- 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);