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SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
On 01/04, Srivatsa Vaddagiri wrote:
>
> On Tue, Dec 19, 2006 at 03:43:19AM +0300, Oleg Nesterov wrote:
> > > Taking workqueue_mutex() unconditionally in flush_workqueue() means
> > > that we'll deadlock if a single-threaded workqueue callback handler calls
> > > flush_workqueue().
> >
> > Well. But flush_workqueue() drops workqueue_mutex before going to sleep ?
>
> ... and acquires it again after woken from sleep. That can be a problem, which
> will lead to the problem described here:
>
> http://lkml.org/lkml/2006/12/7/374
>
> In brief:
>
> keventd thread hotplug thread
> -------------- --------------
>
> run_workqueue()
> |
> work_fn()
> |
> flush_workqueue()
> |
> flush_cpu_workqueue
> | cpu_down()
> mutex_unlock(wq_mutex); |
> (above opens window for hotplug) mutex_lock(wq_mutex);
> | /* bring down cpu */
> wait_for_completition(); notifier(CPU_DEAD, ..)
> | workqueue_cpu_callback
> | cleanup_workqueue_thread
> | kthread_stop()
> |
> |
> mutex_lock(wq_mutex); <- Can deadlock
>
>
> The kthread_stop() will wait for keventd() thread to exit, but keventd()
> is blocked on mutex_lock(wq_mutex) leading to a deadlock.

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?

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?

Oleg.

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