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

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Date:	Wed, 21 May 2014 09:29:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Juri Lelli <juri.lelli@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.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()

On Tue, May 20, 2014 at 01:33:42PM +0400, Kirill Tkhai wrote:
> [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 <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
> ---

thanks Kirill!

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