linux-kernel - Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race with try_to_wake

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Date:	Wed, 3 Sep 2014 11:45:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Kautuk Consul <consul.kautuk@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>,
	David Rientjes <rientjes@...gle.com>,
	Ionut Alexa <ionut.m.alexa@...il.com>,
	Guillaume Morin <guillaume@...infr.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()

On Wed, Sep 03, 2014 at 01:04:36PM +0400, Kirill Tkhai wrote:
> 25.08.2014, 20:01, "Oleg Nesterov" <oleg@...hat.com>:
> > Peter, do you remember another problem with TASK_DEAD we discussed recently?
> > (prev_state == TASK_DEAD detection in finish_task_switch() still looks racy).
> 
> One more problem with task_dead just to mention it here.
> 
> Below is racy with the change of sched_class:
> 
>         if (prev->sched_class->task_dead)
>                 prev->sched_class->task_dead(prev);
> 
> switched_from_dl() does not cancel running timers.

Well, it does a try_to_cancel() but yes, that can fail. Now I suspect
you cannot actually do hrtimer_cancel() from switched_from because its
called with locks held and the timer function will also try and acquire
those locks.

But yes, that appears to be an actual problem indeed.
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