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

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Message-ID: <Pine.LNX.4.64.0801241556130.29058@alien.or.mcafeemobile.com>
Date:	Thu, 24 Jan 2008 15:57:31 -0800 (PST)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Pierre Habouzit <madcoder@...ian.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: epoll and shared fd's

On Thu, 24 Jan 2008, Pierre Habouzit wrote:

> 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

Sure. I'll send Michael Kerrisk and updated statement for the A6 answer in 
the epoll man page.



- Davide


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