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Date: Wed, 27 Jul 2011 23:52:18 +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 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.

Thanks,

	tglx