linux-kernel - Re: [PATCH 1/3] stop_machine: kill __stop

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Message-ID: <1308249362.13240.275.camel@twins>
Date:	Thu, 16 Jun 2011 20:36:02 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Tejun Heo <tj@...nel.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	"x86@...nel.org" <x86@...nel.org>, "mingo@...e.hu" <mingo@...e.hu>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] stop_machine: kill __stop_machine()

On Thu, 2011-06-16 at 20:28 +0200, Tejun Heo wrote:
> Peter, I don't think it's that simple.  get_online_cpus() itself can't
> create circular dependency by itself.  It allows recursing.  The chain
> involves cpu_hotplug_begin() which returns with hotplug mutex held.


Right, its like:

 mutex_lock(&a);
 get_online_cpus();

vs

 cpu_hotplug_begin()
 mutex_lock(&a);


that will really deadlock
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