linux-kernel - Re: [PATCH] sched/dl: Fix race between dl_task_timer() and sched

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Date:	Mon, 19 May 2014 23:31:19 +0400
From:	Kirill Tkhai <tkhai@...dex.ru>
To:	Juri Lelli <juri.lelli@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] sched/dl: Fix race between dl_task_timer() and sched_setaffinity()

19.05.2014, 17:11, "Juri Lelli" <juri.lelli@...il.com>:
> On Sat, 17 May 2014 01:30:03 +0400
> Kirill Tkhai <tkhai@...dex.ru> wrote:
>
>>  The race is in unlocked task_rq() access. In pair with parallel
>>  call of sched_setaffinity() it may be a reason of corruption
>>  of internal rq's data.
>
> Sure, the thing can happen!

[snipped]

>>  @@ -513,9 +513,16 @@ 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;
>
> We could maybe add a comment here, in line with what we have below, to
> document why we need this.

How about this? (I added comment and rewrote changelog).

[PATCH] sched/dl: Fix race between dl_task_timer() and sched_setaffinity()

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.

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(). This case is
an exception.

Signed-off-by: Kirill Tkhai <tkhai@...dex.ru>
CC: Juri Lelli <juri.lelli@...il.com>
CC: Peter Zijlstra <peterz@...radead.org>
CC: Ingo Molnar <mingo@...hat.com>
Cc: <stable@...r.kernel.org> # v3.14

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 800e99b..c0a6921 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 (unlikely(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
--
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