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

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Message-ID: <20140903151848.GA6312@redhat.com>
Date:	Wed, 3 Sep 2014 17:18:48 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 09/03, Peter Zijlstra wrote:
>
> On Wed, Sep 03, 2014 at 03:36:40PM +0200, Oleg Nesterov wrote:
> >
> > 	// Ensure that the previous __set_current_state(RUNNING) can't
> > 	// leak after spin_unlock_wait()
> > 	smp_mb();
> > 	spin_unlock_wait();
> > 	// Another mb to ensure this too can't be reordered with unlock_wait
> > 	set_current_state(TASK_DEAD);
> >
> > What do you think looks better?
>
> spin_unlock_wait() would be a control dependency right? Therefore that
> store could not creep up anyhow.

Hmm. indeed, thanks! This probably means that task_work_run() can use
rmb() instead of mb().

What I can't understand is do we still need a compiler barrier or not.
Probably "in theory yes" ?

Oleg.

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