Arve Hjønnevåg <arve@xxxxxxxxxxx> wrote:
> 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.
True. However, I've been looking into eliminating ep->lock in more
places (maybe entirely)[1].
I don't think the current overhead of RCU in epoll is significant,
either.
[1] I'll be testing Mathieu's wait-free concurrent queue soon:
http://mid.gmane.org/20130311213602.GB9829@Krystal
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