linux-kernel - Re: [PATCH] cpusets: fix deadlock with cpu_down()->cpuset

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Message-ID: <4A70FD26.1010800@cn.fujitsu.com>
Date:	Thu, 30 Jul 2009 09:53:42 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org, Li Zefan <lizf@...fujitsu.com>,
	Miao Xie <miaox@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Gautham R Shenoy <ego@...ibm.com>
Subject: Re: [PATCH] cpusets: fix deadlock with cpu_down()->cpuset_lock()

Oleg Nesterov wrote:
> On 07/29, Oleg Nesterov wrote:
>> I strongly believe the bug does exist, but this patch needs the review
>> from maintainers.
> 
> Yes...
> 
>> IOW, with this patch migration_call(CPU_DEAD) runs without callback_mutex,
>> but kernel/cpuset.c always takes get_online_cpus() before callback_mutex.
> 
> Oh. I'm afraid this is not an option.
> 
> callback_mutex should nest under cgroup_mutex, but cpu hotplu pathes
> take cgroup_mutex under cpu_hotplug->lock. Lockdep won't be happy.
> 
> Oleg.
> 

We have made great effort to remove get_online_cpus() from cgroup_mutex
critical region.

We can migrate the owner of callback_mutex in migration_call(CPU_DEAD)
at first(and then take callback_mutex and migrate others).
It fixes this bug, but it can't help for your
"cpu_hotplug: don't play with current->cpus_allowed" patch.

Lai.

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