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SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
On Thu, Jan 04, 2007 at 05:29:36PM +0300, Oleg Nesterov wrote:
> Thanks, I need to think about this.
>
> However I am not sure I fully understand the problem.
>
> First, this deadlock was not introduced by recent changes (including "single
> threaded flush_workqueue() takes workqueue_mutex too"), yes?

AFAIK this deadlock originated from Andrew's patch here:

http://lkml.org/lkml/2006/12/7/231

(Yes, your patches didnt introduce this. I was just reiterating here my
earlier point that workqueue code is broken of late wrt cpu hotplug).

> Also, it seems to me we have a much more simple scenario for deadlock.
>
> events/0 runs run_workqueue(), work->func() sleeps or takes a preemtion. CPU 0
> dies, keventd thread migrates to another CPU. CPU_DEAD calls kthread_stop() under
> workqueue_mutex and waits for until kevents thread exits. Now, if this work (or
> another work pending on cwq->worklist) takes workqueue_mutex (for example, does
> flush_workqueue) we have a deadlock.
>
> No?

Yes, the above scenario also will cause a deadlock.

I supposed one could avoid the deadlock by having a 'workqueue_mutex_held'
flag and avoid taking the mutex set under some conditions, but IMHO a
more neater solution is to provide a cpu-hotplug lock which works under
all these corner cases. One such proposal was made here:

http://lkml.org/lkml/2006/10/26/65

--
Regards,
vatsa
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