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Date: Fri, 11 Sep 2009 15:53:24 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: Oleg Nesterov <oleg@redhat.com>, Andrew Morton <akpm@linux-foundation.org>,
       Gautham Shenoy <ego@in.ibm.com>, Ingo Molnar <mingo@elte.hu>,
       Jiri Slaby <jirislaby@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>,
       Miao Xie <miaox@cn.fujitsu.com>, Paul Menage <menage@google.com>,
       "Rafael J. Wysocki" <rjw@sisk.pl>,
       Rusty Russell <rusty@rustcorp.com.au>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/3] resend, cpuset/hotplug fixes
References: <20090910192153.GA584@redhat.com>	 <1252615996.7205.99.camel@laptop>  <4AA9F902.4030306@cn.fujitsu.com> <1252654410.7126.1.camel@laptop>
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Peter Zijlstra wrote:
> On Fri, 2009-09-11 at 15:15 +0800, Lai Jiangshan wrote:
>> One other minor thing:
>> Oleg's patch may introduce a trouble in PREEEMPT_RT tree, because
>> spinlock in RT is also mutex. Likely I'm wrong.
> 
> But they have priority-inheritance, hence you cannot create a deadlock
> through preemption. If the kstopmachine thread blocks on a mutex, the
> owner gets boosted to kstopmachine's prio and runs to completion, after
> that kstopmachine continues.
> 

The deadlock is because the owner is at the dead cpu, It's not because
the owner's prio is low. priority-inheritance can't help it.

I think we need to use a raw spinlock.