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

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Date:	Mon, 1 Sep 2014 21:09:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	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, Kirill Tkhai <tkhai@...dex.ru>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()

On Mon, Sep 01, 2014 at 07:58:51PM +0200, Oleg Nesterov wrote:

> However, the very fact that another CPU can look at this task_struct
> means that we still need spin_unlock_wait(). If nothing else to ensure
> that try_to_wake_up()->spin_unlock(pi_lock) won't write into the memory
> we are are going to free.

task_struct is RCU freed, if it still has a 'reference' to the task, it
shouldn't be going 'away', right?

> So I think the comment in do exit should be updated too, and smp_mb()
> should be moved under raw_spin_unlock_wait() but ...
> 
> But. If am right, doesn't this mean we that have even more problems with
> postmortem wakeups??? Why ttwu() can't _start_ after spin_unlock_wait ?

ttwu should bail at: if (!(p->state & state)) goto out; That should
never match with TASK_DEAD.

Either that; or I should go sleep already :-) I shifted 7 hours
yesterday, so I'm still somewhat jet-lagged.
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