linux-kernel - Re: [PATCH 1/2] workqueue: rescuer_thread() processes all pwqs before exit

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Message-ID: <CACvQF53DOj0+sn5EQKLA+mCBdtuceO29gBmH2HCqCOXBmRMWAQ@mail.gmail.com>
Date:	Fri, 18 Apr 2014 00:21:51 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] workqueue: rescuer_thread() processes all pwqs before exit

On Fri, Apr 18, 2014 at 12:08 AM, Tejun Heo <tj@...nel.org> wrote:
> Hello, Lai.
>
> On Fri, Apr 18, 2014 at 12:04:29AM +0800, Lai Jiangshan wrote:
>> > I don't think this is reliable.  What if mayday requests take place
>> > between wq_mayday_lock and kthread_should_stop() check?  We'll
>> > probably need to run through mayday list after checking should_stop.
>>
>> It is destroy_workqueue()'s responsibility to avoid this.
>> destroy_workqueue() should drain all works and refuse any new work queued
>> on the wq before destroy the wq.
>>
>> So since there is no works, there is no new mayday request,
>> and there is no mayday request take place between wq_mayday_lock
>> and kthread_should_stop() check.
>
> Hmmm?  Isn't this the same race condition that you tried to remove by
> relocating the test?  It doesn't matter what destroy_workqueue() does,
> the rescuer may get preempted inbetween and anything can happen
> inbetween including someone maydaying and initiation of
> destroy_workqueue().  Your patch doesn't change the situation at all.
> It can still return with non-empty mayday list.

You are right. We need a additional atomic check.

Thanks,
Lai

>
> Thanks.
>
> --
> tejun
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