Messages in this thread | | | Date | Mon, 24 Aug 2009 19:51:01 +0200 | From | Oleg Nesterov <> | Subject | Re: cancel_delayed_work and its use of del_timer_sync |
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Damn, forgot to update CC, re-sending...
Hi Dmitry,
(add Roland and Stefan)
On 08/23, Dmitry Torokhov wrote: > > I noticed that you went back and forth on using del_timer() vs. > del_timer_sync() in cancel_delayed_work(), finally settling on using > del_timer_sync().
Yes. del_timer() can't work when the user does something like
cancel_delayed_work(dw); flush_workqueue();
kfree(dw);
Perhaps we can add __cancel_delayed_work() which uses del_timer(), most callers don't really need del_timer_sync. I dunno.
> I must say that the fact that cancel_delayed_work() > might sleep
Just in case, it can spin, but not sleep
> (given that there exists cancel_delayed_work_sync) catches a > few drivers writers by surprise. Moreover it makes delayed works > unsuitable in cases when we do want to cancel (so we can reschedule) > work in an interrupt context.
Yep. it is not useable from irq. But only because nobody removed "#ifdef CONFIG_SMP" from set_running_timer(). I guess I should send a patch finally.
> it is possible to have a reschedule_delayed_work() taht would do a > "soft" cancel and [re]submit the work. In the use cases I am concerned > about we don't really care if work is not reliably cancelled, we just > need to be able to schedule it earlier if it has already been scheduled > for execution in some point in the future.
Well. this depends on how "soft" should be that cancel.
Consider the auto-rearming delayed work, its work->func() calls queue_delayed_work(self, BIG_DELAY). The caller of requeue_work(SMALL_DELAY) preempts cwq->thread right after it sets _PENDING.
Now, what should requeue_work() do ? Even if the requeue_work() and work->func() run on different CPUs, in this case requeue_ must spin.
So. It is easy to create requeue_work() which never sleeps/spins, but it can return the error in case it hits the queueing in progress.
Is it OK?
And another question, should it cancel (without sleep/spin) this dwork if the timer has expired, the work is pending, but its ->func() has not started yet?
Oleg.
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