Re: [PATCH 2/3] workqueue: not allow recursion run_workqueue

From: Frederic Weisbecker
Date: Thu Feb 05 2009 - 12:24:56 EST


On Thu, Feb 05, 2009 at 06:01:56PM +0100, 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?


No but CPU#1 can wait for a completion that will never be done, because
CWQ#0 is waiting for CWQ#1.


> 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.
>

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