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

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Message-ID: <1357164511.21409.18486.camel@edumazet-glaptop>
Date:	Wed, 02 Jan 2013 14:08:31 -0800
From:	Eric Dumazet <erdnetdev@...il.com>
To:	Eric Wong <normalperson@...t.net>
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

On Wed, 2013-01-02 at 19:32 +0000, Eric Wong wrote:

> 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.

Yes, and using a mutex for protecting this field while its read from
interrupt context (so without mutex synch help) is why there were races.

Some users rely on barriers included in spin_lock/spin_unlock, others in
explicit barriers, or before your patch pure luck.



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