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SubjectRe: Is adding requeue_delayed_work() a good idea
Now I noticed I forgot to CC Dmitry yesterday...

On 08/24, Roland Dreier wrote:
>
> > > In my particular case it doesn't really matter. In the queued case it
> > > could leave it to run whenever it gets to the head of the workqueue. In
> > > the already running case then I think the timer should be reset. The
> > > main point is that if I do requeue_delayed_work() I want to make sure
> > > the work runs all the way through from the beginning at some point in
> > > the future. The pattern I have in mind is something like:
> > >
> > > spin_lock_irqsave(&mydata_lock);
> > > new_timeout = add_item_to_timeout_list();
> > > requeue_delayed_work(wq, &process_timeout_list_work, new_timeout);
> > > spin_unlock_irqsave(&mydata_lock);
> > >
> > > so if the process_timeout_list_work runs early or twice it doesn't
> > > matter; I just want to make sure that the work runs from the beginning
> > > and sees the new item I added to the list at some point after the
> > > requeue.
> >
> > Hmm. But, asuming that process_timeout_list_work->func() takes mydata_lock
> > too, you can just use queue_delayed_work() ?
> >
> > process_timeout_list_work can't miss new items, queue_delayed_work()
> > can only fail if dwork is pending and its ->func has not started yet.
>
> Maybe I misunderstand the code or misunderstand you,

No, sorry. I misunderstood you (and sorry for delays btw).

I have read "I just want to make sure" above but forgot you also need
to shorten the timeout.

OK, in this case I think we have a simple solution,

// like cancel_delayed_work, but uses del_timer().
// this means, if it returns 0 the timer function may be
// running and the queueing is in progress. The caller
// can't rely on flush_workqueue/etc
static inline int __cancel_delayed_work(struct delayed_work *work)
{
int ret;
ret = del_timer(&work->timer);
if (ret)
work_clear_pending(&work->work);
return ret;
}
Now, you can do

spin_lock_irqsave(&mydata_lock);
new_timeout = add_item_to_timeout_list();

__cancel_delayed_work(&process_timeout_list_work);
queue_delayed_work(wq, &process_timeout_list_work, new_timeout);

spin_unlock_irqsave(&mydata_lock);

If queue_delayed_work() fails, this means that WORK_STRUCT_PENDING is set,
dwork->work is already queued or the queueing is in progress. In both
cases it will run "soon" as if we just called queue_work(&dwork->work).

But this assumes nobody else does queue_delayed_work(dwork, HUGE_DELAY) in
parallel, otherwise we can lose the race and another caller can setup
HUGE_DELAY timeout.

In particular, if process_timeout_list_work->func() itself uses
queue_delay_work() to re-arm itself we can race. Bu t I think it is always
possible to do something to synchronize with work->func, for example
work->func() can re-arm itself _before_ it scans timeout_list (under the
same lock). This way, if re-queue code above fails because work->func()
wins, work->func() must see the new additions to timeout_list.

Can this work for you?

Oleg.



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