Re: epoll: possible bug from wakeup_source activation

[Eric Wong]

Eric Wong <normalperson@xxxxxxxx> wrote:
> Arve HjÃnnevÃg <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.
> 
> Perhaps RCU can be used?  I've no experience with RCU, but I've been
> meaning to get acquainted with RCU.

The following is lightly tested, at least I haven't gotten lockdep
to complain.

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