From mboxrd@z Thu Jan  1 00:00:00 1970
From: Eric Dumazet <erdnetdev@gmail.com>
Subject: Re: [PATCH] epoll: prevent missed events on EPOLL_CTL_MOD
Date: Wed, 02 Jan 2013 14:08:31 -0800
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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To: Eric Wong <normalperson@yhbt.net>
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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.