Re: epoll and shared fd's

[Michael Kerrisk <mtk.manpages@googlemail.com>]

Following up after quite some time:

Davide Libenzi wrote:
> On Sat, 26 Jan 2008, Michael Kerrisk wrote:
> 
>> On Jan 25, 2008 12:57 AM, Davide Libenzi <davidel@xmailserver.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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> 
> Something like the one below ...
> 
> 
> - Davide
> 
> 
> 
> --- epoll.4	2008-01-26 12:58:21.000000000 -0800
> +++ epoll.4.new	2008-01-26 13:06:36.000000000 -0800
> @@ -285,7 +285,19 @@
>  sets automatically?
>  .TP
>  .B A6
> -Yes.
> +A file descriptor is the userspace counterpart of an internal kernel handle.
> +Every time a process calls functions liks
> +.BR dup (2),
> +.BR dup2 (2)
> +or
> +.BR fork (2),
> +a new file descriptor referring to the same internal kernel handle is
> +created. The internal kernel handle remains alive until all the userspace
> +file descriptors have been closed.
> +The
> +.BR epoll (4)
> +interface automatically removes the internal kernel handle from the set,
> +once all the file descriptor instances have been closed.
>  .TP
>  .B Q7
>  If more than one event occurs between

Davide,

Two points.

a) I did a

s/internal kernel handle/open file description/

since that is the POSIX term for the internal handle.

b) It seems to me that you text doesn't quite make the point explicit
enough.  I've tried to rewrite it; could you please check:

       A6     Yes, but be aware of the following point.  A  file
              descriptor is a reference to an open file descrip-
              tion (see  open(2)).   Whenever  a  descriptor  is
              duplicated  via dup(2), dup2(2), fcntl(2) F_DUPFD,
              or fork(2), a new file descriptor referring to the
              same  open  file  description is created.  An open
              file description continues to exist until all file
              descriptors referring to it have been closed.  The
              epoll  interface  automatically  removes  a   file
              descriptor  from  an  epoll set only after all the
              file descriptors referring to the underlying  open
              file  handle  have  been  closed.  This means that
              even after a file descriptor that is  part  of  an
              epoll  set has been closed, events may be reported
              for that file descriptor if other file descriptors
              referring  to the same underlying file description
              remain open.

Does that seem okay?  I plan to include the text in man-pages-2.79.

Was there some reason why removing a file descriptor couldn't have been
made to do the "expected" thing (i.e., remove notifications for that file
descriptor, regardless of whether the underlying file description remains
open)?

Cheers,

Michael

-- 
Michael Kerrisk
Maintainer of the Linux man-pages project
http://www.kernel.org/doc/man-pages/
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