linux-kernel - RE: epoll and closed file descriptors

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Message-ID: <024601ca372d$09ad9fb0$1d08df10$@com>
Date:	Wed, 16 Sep 2009 17:23:09 -0700
From:	"Gilad Benjamini" <gilad@...ornetworks.com>
To:	"'Davide Libenzi'" <davidel@...ilserver.org>
Cc:	"'Linux Kernel Mailing List'" <linux-kernel@...r.kernel.org>
Subject: RE: epoll and closed file descriptors

Davide wrote:
> On Wed, 16 Sep 2009, Gilad Benjamini wrote:
> 
> > I am running repeatedly into a scenario  where epoll notifies
> userland of
> > events on a closed file descriptor.
> > I am running a single thread application, on a single CPU machine so
> > multiple threads isn't the issue.
> >
> > A sample set of events that I have seen
> > - File descriptor (13) for a socket is closed
> > - epoll_wait returns with no events.
> > - Several epoll related calls happen
> > - More than 20 seconds after the "close", epoll_wait finds an event
> on fd 13
> > with EPOLLIN|EPOLLERR|EPOLLHUP.
> > - epoll_wait continues to report this event
> 
> Epoll removes the fd from its container, when the last instance of the
> underlying kernel file pointer is released (or when you explicitly
> remove it with epoll_ctl(EPOLL_CTL_DEL)).
> If you continue to get the event, it means that someone else has an
> instance of the socket (that, looking at the events, saw a shutdown)
> open,
> by hence keeping the kernel object alive.
> If you don't want to see the events, just remove the socket from the
> epoll
> set before closing.
> Or, you remove the socket the first time you see an EPOLLHUP.
> 
> 
> 
> - Davide

I would, but epoll is preventing me from doing so.
Early in sys_epoll_ctl there are these lines

  file = fget(epfd);
  if (!file)
    goto error_return;

Leaving me in a kind of dead lock

Gilad

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