linux-kernel - Re: migrate_disable() race with cpu hotplug?

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Message-ID: <alpine.LFD.2.02.1107272350560.2660@ionos>
Date:	Wed, 27 Jul 2011 23:52:18 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Yong Zhang <yong.zhang0@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: migrate_disable() race with cpu hotplug?

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
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