linux-kernel - Re: epoll and shared fd's

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Message-ID: <Pine.LNX.4.64.0801181257240.24685@alien.or.mcafeemobile.com>
Date:	Fri, 18 Jan 2008 13:10:18 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Pierre Habouzit <madcoder@...ian.org>
cc:	linux-kernel@...r.kernel.org
Subject: Re: epoll and shared fd's

On Fri, 18 Jan 2008, Pierre Habouzit wrote:

>   Hi,
> 
>   I just came across a strange behavior of epoll that seems to
> contradict the documentation. Here is what happens:
> 
> * I have two processes P1 and P2, P1 accept()s connections, and send the
>   resulting file descriptors to P2 through a unix socket.
> 
> * P2 registers the received socket in his epollfd.
> 
>   [time passes]
> 
> * P2 is done with the socket and closes it
> 
> * P2 gets events for the socket again !
> 
> 
>   Though the documentation says that if a process closes a file
> descriptor, it gets unregistered. And yes I'm sure that P2 doens't dup()
> the file descriptor. Though (because of a bug) it was still open in
> P1[0], hence the referenced socket still live at the kernel level.
> 
>   Of course the userland workaround is to force the EPOLL_CTL_DEL before
> the close, which I now do, but costs me a syscall where I wanted to
> spare one :|

For epoll, a close is when the kernel file* is released (that is, when all 
its instances are gone).
We could put a special handling in filp_close(), but I don't think is a 
good idea, and we're better live with the current behaviour.



- Davide


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