linux-kernel - Re: epoll: possible bug from wakeup

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Date:	Mon, 11 Mar 2013 17:29:00 -0700
From:	Arve Hjønnevåg <arve@...roid.com>
To:	Eric Wong <normalperson@...t.net>
Cc:	NeilBrown <neilb@...e.de>, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org,
	Davide Libenzi <davidel@...ilserver.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: epoll: possible bug from wakeup_source activation

On Mon, Mar 11, 2013 at 5:17 PM, Eric Wong <normalperson@...t.net> wrote:
> Arve Hjønnevåg <arve@...roid.com> wrote:
>> On Fri, Mar 8, 2013 at 11:10 PM, Eric Wong <normalperson@...t.net> wrote:
>> > Arve Hjønnevåg <arve@...roid.com> wrote:
>> >> On Fri, Mar 8, 2013 at 12:49 PM, Eric Wong <normalperson@...t.net> 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

OK, but is there any way you could use the same locking scheme for the
wakeup_source and the queue?

-- 
Arve Hjønnevåg
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