From 0f397f2c90ce68821ee864c2c53baafe78de765d Mon Sep 17 00:00:00 2001 From: Kirill Tkhai Date: Tue, 20 May 2014 13:33:42 +0400 Subject: [PATCH] sched/dl: Fix race in dl_task_timer() Throttled task is still on rq, and it may be moved to other cpu if user is playing with sched_setaffinity(). Therefore, unlocked task_rq() access makes the race. Juri Lelli reports he got this race when dl_bandwidth_enabled() was not set. Other thing, pointed by Peter Zijlstra: "Now I suppose the problem can still actually happen when you change the root domain and trigger a effective affinity change that way". To fix that we do the same as made in __task_rq_lock(). We do not use __task_rq_lock() itself, because it has a useful lockdep check, which is not correct in case of dl_task_timer(). We do not need pi_lock locked here. This case is an exception (PeterZ): "The only reason we don't strictly need ->pi_lock now is because we're guaranteed to have p->state == TASK_RUNNING here and are thus free of ttwu races". Signed-off-by: Kirill Tkhai Signed-off-by: Peter Zijlstra Cc: # v3.14+ Cc: Linus Torvalds Link: http://lkml.kernel.org/r/3056991400578422@web14g.yandex.ru Signed-off-by: Ingo Molnar --- kernel/sched/deadline.c | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-) diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c index 800e99b99075..14bc348ba3b4 100644 --- a/kernel/sched/deadline.c +++ b/kernel/sched/deadline.c @@ -513,9 +513,17 @@ static enum hrtimer_restart dl_task_timer(struct hrtimer *timer) struct sched_dl_entity, dl_timer); struct task_struct *p = dl_task_of(dl_se); - struct rq *rq = task_rq(p); + struct rq *rq; +again: + rq = task_rq(p); raw_spin_lock(&rq->lock); + if (rq != task_rq(p)) { + /* Task was moved, retrying. */ + raw_spin_unlock(&rq->lock); + goto again; + } + /* * We need to take care of a possible races here. In fact, the * task might have changed its scheduling policy to something