linux-kernel - Re: epoll with ONESHOT possibly fails to deliver events

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Message-ID: <50CA6E04.1070105@turmel.org>
Date:	Thu, 13 Dec 2012 19:08:36 -0500
From:	Phil Turmel <philip@...mel.org>
To:	Eric Wong <normalperson@...t.net>
CC:	Andreas Voellmy <andreas.voellmy@...e.edu>,
	viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: epoll with ONESHOT possibly fails to deliver events

On 12/13/2012 04:32 AM, Eric Wong wrote:
> Andreas Voellmy <andreas.voellmy@...e.edu> wrote:

[trim /]

>>> Another thread, distinct from all of the threads serving particular
>>> sockets, is perfoming epoll_wait calls. When sockets are returned as
>>> being ready from an epoll_wait call, the thread signals to the
>>> condition variable for the socket.
> 
> Perhaps there is a bug in the way your epoll_wait thread
> uses the condition variable to notify other threads?

Have you considered the possibility that data is arriving between
epoll_ctl and pthread_cond_wait ?  If your monitoring thread returns
from epoll_wait within this race window, it will call
pthread_cond_signal while the first thread is not yet waiting for it.
With the one-shot flag, the next iteration of epoll_wait won't see that
socket's new data.

Phil
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