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SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
On 01/04, Andrew Morton wrote:
>
> On Thu, 4 Jan 2007 17:29:36 +0300
> Oleg Nesterov <oleg@tv-sign.ru> wrote:
>
> > > 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.
>
> This?
>
>
> --- a/kernel/workqueue.c~flush_workqueue-use-preempt_disable-to-hold-off-cpu-hotplug
> +++ a/kernel/workqueue.c
> @@ -419,18 +419,22 @@ static void flush_cpu_workqueue(struct c
> * Probably keventd trying to flush its own queue. So simply run
> * it by hand rather than deadlocking.
> */
> - mutex_unlock(&workqueue_mutex);
> + preempt_enable();

Ah, (looking at _cpu_down()->stop_machine()), so preempt_disable() not only "pins"
the current CPU, it blocks cpu_down(), yes ???

I guess this should work then. I'll try to re-check this code on weekend.

Oleg.

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