linux-kernel - Re: [PATCH 2/3] workqueue: not allow recursion run

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Date:	Fri, 6 Feb 2009 17:46:49 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Eric Dumazet <dada1@...mosbay.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

On 02/06, Lai Jiangshan wrote:
>
> Oleg Nesterov wrote:
> > On 02/05, Lai Jiangshan wrote:
> >> DEADLOCK EXAMPLE for explain my above option:
> >>
> >> (work_func0() and work_func1() are work callback, and they
> >> calls flush_workqueue())
> >>
> >> CPU#0					CPU#1
> >> run_workqueue()                         run_workqueue()
> >>   work_func0()                            work_func1()
> >>     flush_workqueue()                       flush_workqueue()
> >>       flush_cpu_workqueue(0)                  .
> >>       flush_cpu_workqueue(cpu#1)              flush_cpu_workqueue(cpu#0)
> >>         waiting work_func1() in cpu#1           waiting work_func0 in cpu#0
> >>
> >> DEADLOCK!
> >
> > I am not sure. Note that when work_func0() calls run_workqueue(),
> > it will clear cwq->current_work, so another flush_ on CPU#1 will
> > not wait for work_func0, no?
>
> cwq->current_work is changed only when
> !list_empty(&cwq->worklist)
> in run_workqueue().
>
> so cwq->current_work may not be changed.

Ah, indeed.

Thanks for correcting me!

Oleg.

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