Re: epoll: possible bug from wakeup_source activation

From: Arve Hjønnevåg
Date: Mon Mar 11 2013 - 19:38:00 EST

On Fri, Mar 8, 2013 at 11:10 PM, Eric Wong <normalperson@xxxxxxxx> wrote:
> Arve Hjønnevåg <arve@xxxxxxxxxxx> wrote:
>> On Fri, Mar 8, 2013 at 12:49 PM, Eric Wong <normalperson@xxxxxxxx> wrote:
>> > What happens if ep_modify calls ep_destroy_wakeup_source
>> > while __pm_stay_awake is running on the same epi->ws?
>> Yes, that looks like a problem. I think calling
>> ep_destroy_wakeup_source with ep->lock held should fix that. It is not
>> clear how useful changing EPOLLWAKEUP in ep_modify is, so
>> alternatively we could remove that feature and instead only allow it
>> to be set in ep_insert.
> ep->lock would work, but ep->lock is already a source of heavy
> contention in my multithreaded+epoll webservers.

This should not have any significant impact on that since you would be
adding a lock to a code path that is, as far as I know, unused.

> Perhaps RCU can be used? I've no experience with RCU, but I've been
> meaning to get acquainted with RCU.

That adds code to the common path however. The wakeup_source is not
touch without holding one of the locks so holding both locks before
deleting it seems like a simpler solution.

> Another possible solution is to only use ep->ws and add an atomic
> counter to ep; so __pm_relax(ep->ws) is only called when the atomic
> counter reaches zero.

Arve Hjønnevåg
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