linux-kernel - Re: epoll design problems with common fork/exec patterns

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Date:	Sun, 2 Mar 2008 13:44:53 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Sam Varshavchik <mrsam@...rier-mta.com>
cc:	linux-man@...r.kernel.org,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Chris ¥¯ Heath <chris@...thens.co.nz>,
	David Schwartz <davids@...master.com>, dada1@...mosbay.com,
	Linux-Kernel@...r. Kernel. Org 
	<linux-kernel@...r.kernel.org>
Subject: Re: epoll design problems with common fork/exec patterns

On Sun, 2 Mar 2008, Sam Varshavchik wrote:

> Hijacking this epoll thread, the following related question occurs to me:
> 
> #Q8
> #    Does an operation on a file descriptor affect the already collected but
> #not yet reported events?
> #
> #A8
> #    You can do two operations on an existing file descriptor. Remove would
> #be meaningless for this case. Modify will re-read available I/O.
> 
> Why is EPOLL_CTL_DEL considered meaningless? A process is wrapping up its
> business and is preparing to remove the fd from the epoll set, and then close
> the file descriptor itself. In the meantime, the fd became readable, and a
> POLLIN event gets collected. So, what happens to the collected event, when the
> EPOLL_CTL_DEL operation is made?

Any epoll_wait() done after the POLLIN and before the EPOLL_CTL_DEL, will 
show up. After the EPOLL_CTL_DEL, of course, no events will be reported.



- Davide


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