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

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Date:	Thu, 4 Sep 2014 09:15:26 +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>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>
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 05:18:48PM +0200, Oleg Nesterov wrote:
> 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" ?

Yes, this is where I'm forever in doubt as well. The worry is the
compiler reordering things, but I'm not sure how it would do that in
this case, then again, I've been shown to not be creative enough in
these cases many times before.

Paul might know, he's had much more exposure to compiler people.
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