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

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Date:	Sat, 9 Mar 2013 07:10:37 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Arve Hjønnevåg <arve@...roid.com>
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

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.

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

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.
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