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: <20140904170308.GH5001@linux.vnet.ibm.com>
Date:	Thu, 4 Sep 2014 10:03:08 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	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 Thu, Sep 04, 2014 at 09:15:26AM +0200, Peter Zijlstra wrote:
> 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.

Well, if we are talking about the code sequence above, spin_unlock_wait()
does reads followed by a conditional.  And set_current_state() does
a write.  The one thing that might be missing is that for this to work is
that Alpha might need an ACCESS_ONCE() to avoid read reordering.

(Yes, ACCESS_ONCE() is required for the other architectures to meet
the letter of the law in memory-barriers.txt, but if you know that
your particular architecture and compiler won't mess you up, you
have more freedom in your arch-specific code.)

							Thanx, Paul

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