linux-kernel - Re: [PATCH 1/6] kill the broken and deadlockable cpuset_lock/cpuset_cpus_allowed

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Date:	Thu, 25 Mar 2010 11:00:59 +0800
From:	Miao Xie <miaox@...fujitsu.com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	Ben Blum <bblum@...gle.com>, Jiri Slaby <jirislaby@...il.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Paul Menage <menage@...gle.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>, Tejun Heo <tj@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] kill the broken and deadlockable	cpuset_lock/cpuset_cpus_allowed_locked
 code

on 2010-3-15 17:10, Oleg Nesterov wrote:
> This patch just states the fact the cpusets/cpuhotplug interaction is
> broken and removes the deadlockable code which only pretends to work.
> 
> - cpuset_lock() doesn't really work. It is needed for
>   cpuset_cpus_allowed_locked() but we can't take this lock in
>   try_to_wake_up()->select_fallback_rq() path.
> 
> - cpuset_lock() is deadlockable. Suppose that a task T bound to CPU takes
>   callback_mutex. If cpu_down(CPU) happens before T drops callback_mutex
>   stop_machine() preempts T, then migration_call(CPU_DEAD) tries to take
>   cpuset_lock() and hangs forever because CPU is already dead and thus
>   T can't be scheduled.

The problem what you said don't exist, because the kernel already move T to
the active cpu when preparing to turn off a CPU.

Thanks!
Miao

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