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Date: Fri, 06 Feb 2009 09:20:21 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: Oleg Nesterov <oleg@redhat.com>
CC: Peter Zijlstra <peterz@infradead.org>,
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       Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Eric Dumazet <dada1@cosmosbay.com>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue
References: <497838F0.7020408@cn.fujitsu.com> <20090122093046.GC5891@nowhere> <20090122093649.GD24758@elte.hu> <c62985530901220306p78ea541cs28912a844297b304@mail.gmail.com> <1232622615.4890.114.camel@laptop> <498AA0F1.2030003@cn.fujitsu.com> <20090205170156.GA25517@redhat.com>
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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.

> 
> But anyway. Nobody argues, "if (cwq->thread == current) {...}" code in
> flush_cpu_workqueue() is bad and should die. Otrherwise, we should
> fix the lockdep warning ;)
> 
> The only problem: if we still have the users of this hack, they will
> deadlock. But perhaps it is time to fix them.
> 
> And, if it was not clear, I do agree with this change. And Peter
> seems to agree as well.
> 
> Oleg.
> 
> 
> 
>