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SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
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();
+ /*
+ * We can still touch *cwq here because we are keventd, and
+ * hot-unplug will be waiting us to exit.
+ */
run_workqueue(cwq);
- mutex_lock(&workqueue_mutex);
+ preempt_disable();
} else {
struct wq_barrier barr;

init_wq_barrier(&barr);
__queue_work(cwq, &barr.work);

- mutex_unlock(&workqueue_mutex);
+ preempt_enable(); /* Can no longer touch *cwq */
wait_for_completion(&barr.done);
- mutex_lock(&workqueue_mutex);
+ preempt_disable();
}
}

@@ -449,7 +453,7 @@ static void flush_cpu_workqueue(struct c
*/
void fastcall flush_workqueue(struct workqueue_struct *wq)
{
- mutex_lock(&workqueue_mutex);
+ preempt_disable(); /* CPU hotplug */
if (is_single_threaded(wq)) {
/* Always use first cpu's area. */
flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
@@ -459,7 +463,7 @@ void fastcall flush_workqueue(struct wor
for_each_online_cpu(cpu)
flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
}
- mutex_unlock(&workqueue_mutex);
+ preempt_enable();
}
EXPORT_SYMBOL_GPL(flush_workqueue);

_
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