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SubjectRe: [PATCH, RFC] reimplement flush_workqueue()
On 01/04, Srivatsa Vaddagiri wrote:
>
> 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

I don't think so. The core problem is not that we are doing unlock/sleep/lock
with this patch. The thing is: work->func() can't take wq_mutex (and thus use
flush_work/workqueue) because it is possible that CPU_DEAD holds this mutex
and waits for us to complete(kthread_stop_info). I believe this bug is old.

> (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.

Ok, thanks for acknowledgement.

> I supposed one could avoid the deadlock by having a 'workqueue_mutex_held'
> flag and avoid taking the mutex set under some conditions,

I am thinking about the same right now. Probably we can do something like this:


int xxx_lock(void)
{
for (;;) {
if (mutex_trylock(wq_mutex))
return 1;
// the owner of wq_mutex sleeps, we can proceed
if (kthread_should_stop())
return 0;
}
}
void xxx_unlock(int yesno)
{
if (yesno)
mutex_unlock(wq_mutext);
}
and then do

locked = xxx_lock();
...
xxx_unlock(locked);

in flush_xxx() instead of plain lock/unlock.

Yes, ugly. I'll try to do something else on weekend.

> 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

I'll take a look later, thanks.

Oleg.

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