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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