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

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Message-ID: <20080124084001.GB7356@artemis.madism.org>
Date:	Thu, 24 Jan 2008 09:40:01 +0100
From:	Pierre Habouzit <madcoder@...ian.org>
To:	Davide Libenzi <davidel@...ilserver.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: epoll and shared fd's

On Fri, Jan 18, 2008 at 09:10:18PM +0000, Davide Libenzi wrote:
> 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.

  Okay, maybe updating the linux manpages to be more clear about that is
the way to go then. Thanks

-- 
·O·  Pierre Habouzit
··O                                                madcoder@...ian.org
OOO                                                http://www.madism.org

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