Re: [Bug #14388] keyboard under X with 2.6.31

Oleg Nesterov
2009-10-15T19:00:24Z

On 10/16, OGAWA Hirofumi wrote:
>
> > +void flush_delayed_work(struct delayed_work *dwork)
> > +{
> > + if (del_timer(&dwork->timer)) {
> > + struct cpu_workqueue_struct *cwq;
> > + cwq = wq_per_cpu(keventd_wq, get_cpu());
> > + __queue_work(cwq, &dwork->work);
> > + put_cpu();
> > + }
> > + flush_work(&dwork->work);
> > +}
> > +EXPORT_SYMBOL(flush_delayed_work);
> > +
> > +/**
>
> Sorry if I'm missing the point. Doesn't this have (possible) race with
> schedule_delayed_work() (i.e. by tty writer)?
>
> cpu0 cpu1
>
> if (del_timer(&dwork->timer)) {

If dwork->timer is pending - _PENDING must be set.
If del_timer() succeeds, nobody else can clear this bit.

> // cpu0 doesn't set _PENDING
> schedule_delayed_work()

and in this case schedule_delayed_work()->queue_delayed_work_on()
can't succeed because it does test_and_set_bit(_PENDING).

But. Since this helper was merged, I think it should use del_timer_sync()
to be correct. Yes, it is slower, but otherwise flush is racy.

And I think it should return a bolean to match flush_work(). IOW,

int flush_delayed_work(struct delayed_work *dwork)
{
int requeued = false;

if (del_timer(&dwork->timer)) {
struct cpu_workqueue_struct *cwq;
cwq = wq_per_cpu(keventd_wq, get_cpu());
__queue_work(cwq, &dwork->work);
put_cpu();

requeued = true;
}

return flush_work(&dwork->work) || requeued;
}

Not that I think this is terribly important, but still.

I'll send the patch.

Oleg.