On Fri, Feb 26, 2016 at 04:38:18PM +0100, Petr Mladek wrote:
> On Thu 2016-02-25 13:59:32, Peter Zijlstra wrote:
> > On Wed, Feb 24, 2016 at 05:18:05PM +0100, Petr Mladek wrote:
> > > @@ -770,7 +782,22 @@ void delayed_kthread_work_timer_fn(unsigned long __data)
> > > if (WARN_ON_ONCE(!worker))
> > > return;
> > >
> > > - spin_lock(&worker->lock);
> > > + /*
> > > + * We might be unable to take the lock if someone is trying to
> > > + * cancel this work and calls del_timer_sync() when this callback
> > > + * has already been removed from the timer list.
> > > + */
> > > + while (!spin_trylock(&worker->lock)) {
> > > + /*
> > > + * Busy wait with spin_is_locked() to avoid cache bouncing.
> > > + * Break when canceling is set to avoid a deadlock.
> > > + */
> > > + do {
> > > + if (work->canceling)
> > > + return;
> > > + cpu_relax();
> > > + } while (spin_is_locked(&worker->lock));
> > > + }
> > > /* Work must not be used with more workers, see queue_kthread_work(). */
> > > WARN_ON_ONCE(work->worker != worker);
> > >
> >
> > This is pretty vile; why can't you drop the lock over del_timer_sync() ?
>
> We would need to take the lock later and check if nobody has set the timer
> again in the meantime.
Well, if ->cancelling is !0, nobody should be re-queueing, re-arming
timers etc.., right?
And since you do add_timer() while holding the spinlock, this should all
work out, no?
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