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: <20140903144450.GB7083@twins.programming.kicks-ass.net>
Date:	Wed, 3 Sep 2014 16:44:50 +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 Wed, Sep 03, 2014 at 03:36:40PM +0200, Oleg Nesterov wrote:
> Peter, sorry for slow responses.

No worries, I'm not entirely fast myself. Slept most of the day :-)

> Ah, I simply do not know what is cheaper, even on x86. Well, we need
> to enable/disable irqs, but again I do not really know how much does
> this cost. 

Ah good point about that IRQ thing, yes that's horribly expensive.

> I can even say what (imo) looks better, lock/unlock above or
> 
> 	// 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.
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