Messages in this thread |  | | | Date | Mon, 7 May 2007 14:34:20 +0400 | | From | Oleg Nesterov <> | | Subject | Re: [PATCH] make-cancel_rearming_delayed_work-reliable-fix |
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On 05/07, Jarek Poplawski wrote: > > There is a lot of new things in the final version of this > patch. I guess, there was no such problem in the previous > version.
No, this is basically the same patch + re-check-cwq-after-lock, the latter is mostly needed to prevent racing with CPU-hotplug.
> I can also see you have new doubts about usefulness, which > I cannot understand: > - even if there are some slowdowns, where does it matter? > - the "old" method uses only one method of cancelling, i.e. > del_timer, not trying to stop requeuing or to remove from > the queue; it seems to be effective only with long delayed > timers, and its real problems are probably mostly invisible.
The slowdown is small, changelog mentions it just to be "fair".
I am not happy with the complication this patch adds, mostly I hate this smb_wmb() in insert_work(). I have an idea how to remove it later, but this needs another patch not related to workqueue.c.
> BTW, I'm still not convinced all additions are needed: > the "old" cancel_rearming_ doesn't care about checking > or waiting on anything after del_timer positive.
It would be very strange to do wait_on_work() only in case when del_timer() failed. This way we still need to do cancel_work_sync() after cancel_rearming_delayed_work(), but only when del_timer() failed, ugly. Note also that wait_on_work() does not sleep if work->func() is not running.
Also, consider this callback:
void work_handler(struct work_struct *w) { struct delayed_work dw = container_of(...); queue_delayed_work(dw, delay);
// <------------- cancel_rearming_delayed_work() cancel_delayed_work(dw); queue_delayed_work(dw, another_delay); } Yes, this is strange and ugly. But correct! The current version (before this patch) can't cancel this delayed_work. The new implementation works correctly. So I think it is far better to do wait_on_work() unconditionally.
> PS: I'll try to check this all in the evening and will > write tomorrow, if found something interesting.
Yes, please!
Oleg.
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