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Date: Thu, 28 Jul 2011 08:55:07 +0200 (CEST)
From: Thomas Gleixner <tglx@linutronix.de>
To: Yong Zhang <yong.zhang0@gmail.com>
cc: LKML <linux-kernel@vger.kernel.org>,
        linux-rt-users <linux-rt-users@vger.kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>
Subject: Re: migrate_disable() race with cpu hotplug?
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On Thu, 28 Jul 2011, Yong Zhang wrote:

> On Wed, Jul 27, 2011 at 11:52:18PM +0200, Thomas Gleixner wrote:
> > On Tue, 26 Jul 2011, Yong Zhang wrote:
> > 
> > > When reading the code, I'm afraid there is a race between migrate_disable()
> > > and cpu hotplug. A scenario will like below:
> > > 
> > > 	CPU0			CPU1
> > > _cpu_down();
> > >   cpu_unplug_begin();
> > >     wait_for_completion()
> > >   			sync_unplug_thread();
> > > 			  complete();
> > > 			race_window? /*
> > > 				      * migrate_disable() will
> > > 				      * not take effect since
> > > 				      * hotplug is in progress
> > > 				      */
> > 
> > Rightfully so. The caller will just block on the cpu_hotplug.lock
> > mutex until the unplug operation will be done.
> 
> Yup, just notice the mutex_lock/mutex_unlock in pin_current_cpu().
> 
> But if the caller block on mutex_lock() then waked up, it's possible
> that it's been migrated to another cpu. So in the 'retry' loop, we
> should reget hotplug_pcp.

Ooops, that was the original plan. I somehow managed to fatfinger
that. Good catch.
 
Thanks,

	tglx