linux-kernel - Re: [PATCH] epoll: prevent missed events on EPOLL_CTL

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Message-ID: <20130102193227.GA13566@dcvr.yhbt.net>
Date:	Wed, 2 Jan 2013 19:32:27 +0000
From:	Eric Wong <normalperson@...t.net>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Hans Verkuil <hans.verkuil@...co.com>,
	Jiri Olsa <jolsa@...hat.com>, Jonathan Corbet <corbet@....net>,
	Al Viro <viro@...iv.linux.org.uk>,
	Davide Libenzi <davidel@...ilserver.org>,
	Hans de Goede <hdegoede@...hat.com>,
	Mauro Carvalho Chehab <mchehab@...radead.org>,
	David Miller <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andreas Voellmy <andreas.voellmy@...e.edu>,
	"Junchang(Jason) Wang" <junchang.wang@...e.edu>,
	Network Development <netdev@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD

Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2013-01-02 at 18:40 +0000, Eric Wong wrote:
> > Eric Dumazet <eric.dumazet@...il.com> wrote:
> > > It seems the real problem is the epi->event.events = event->events;
> > > which is done without taking ep->lock
> > 
> > Yes.  I am hoping it is possible to do it without a lock there,
> > but your change is more obviously correct.
> > 
> > > While a smp_mb() could reduce the race window, I believe there is still
> > > a race, and the following patch would close it.
> > 
> > I'm not an experienced kernel hacker, can you describe where the race
> > would be?
> 
> It would be for example in ep_send_events_proc() doing :
> 
> if (epi->event.events & EPOLLONESHOT)
>     epi->event.events &= EP_PRIVATE_BITS;
> 
> And this could happen at the same time.

That modification in ep_send_events_proc() is protected by ep->mtx
(as is ep_modify()), though.  Maybe there are other places, but I
don't see it.
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