netdev - Re: [PATCH 1/2] workqueue: Catch more locking problems with flush

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Message-ID: <CAM2zO=DEmR6jpwW9OZuD-tHchhb-mtSTv2_uWV=4wG8JK7o55g@mail.gmail.com>
Date:	Sat, 21 Apr 2012 08:32:55 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
	netdev@...r.kernel.org, Ben Dooks <ben-linux@...ff.org>
Subject: Re: [PATCH 1/2] workqueue: Catch more locking problems with flush_work()

On Fri, Apr 20, 2012 at 4:32 PM, Yong Zhang <yong.zhang0@...il.com> wrote:
> On Fri, Apr 20, 2012 at 01:18:19AM -0700, Stephen Boyd wrote:
>> On 4/20/2012 12:18 AM, Yong Zhang wrote:
>> > On Thu, Apr 19, 2012 at 11:26:47PM -0700, Stephen Boyd wrote:
>> >> complain in the case where the work is not queued. That case is not a
>> >> false positive. We will get a lockdep warning if the work is running
>> > IIRC, flush_work() is just a nop when a work is not queued nor running.
>>
>> Agreed, but it's better to always print a lockdep warning instead of
>> only when the deadlock is going to occur.
>
> It will IMHO.

After reconsidering this issue, I recognize I'm wrong from the beginning.
My suggestion will only warn on the real deadlock. It doesn't follow
what lockdep want to achieve. Sorry for that.

BTW, your latest patch should work :)

Thanks,
Yong

>
>>
>> >
>> >> (when start_flush_work() returns true) solely with the
>> >> lock_map_acquire() on cwq->wq->lockdep_map.
>> > Yeah, that is the point we use lockdep to detect deadlock for workqueue.
>> >
>> > But when looking at start_flush_work(), for some case
>> > !(cwq->wq->saved_max_active == 1 || cwq->wq->flags & WQ_RESCUER),
>> > lock_map_acquire_read() is called, but recursive read is not added to
>> > the chain list. So when lock_map_acquire_read(&cwq->wq->lockdep_map)
>> > is called, deadlock will not be detected. I hope you don't hit that
>> > special case.
>>
>> Hmm. Originally I had what you suggested in my patch but I left it out
>> because I wasn't sure if it would cause false positives.
>> Do you see any
>> possibility for false positives?
>
> No, I don't. My test indeed show nothing (just build and boot).
>
>>I'll add it back in if not.
>
> It's great if you can try it :)
>
> Thanks,
> Yong



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