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Date: Wed, 3 Sep 2014 16:44:50 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Kautuk Consul <consul.kautuk@gmail.com>, Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.cz>, David Rientjes <rientjes@google.com>,
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        Guillaume Morin <guillaume@morinfr.org>, linux-kernel@vger.kernel.org,
        Kirill Tkhai <tkhai@yandex.ru>
Subject: Re: [PATCH 1/1] do_exit(): Solve possibility of BUG() due to race
 with try_to_wake_up()
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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.