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

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Message-ID: <517f3f820801252337lcd4b0a0hde7359dc46a947dd@mail.gmail.com>
Date:	Sat, 26 Jan 2008 08:37:41 +0100
From:	"Michael Kerrisk" <mtk.manpages@...il.com>
To:	"Davide Libenzi" <davidel@...ilserver.org>
Cc:	"Pierre Habouzit" <madcoder@...ian.org>,
	"Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>,
	mtk/manpages@...il.com
Subject: Re: epoll and shared fd's

On Jan 25, 2008 12:57 AM, Davide Libenzi <davidel@...ilserver.org> wrote:
>
> 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.

Thanks Davide -- yes please send me a patch.
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