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SubjectRe: epoll and shared fd's
On Tue, 26 Feb 2008, Michael Kerrisk wrote:

> 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.
> >> --
> >> To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> >> the body of a message to majordomo@vger.kernel.org
> >> More majordomo info at http://vger.kernel.org/majordomo-info.html
> >> Please read the FAQ at http://www.tux.org/lkml/
> >>
> >
> > 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.

I agree with Bodo, it is kinda confusing. The name "open file description",
even though POSIX, looks very similar to "file descriptor".
I honestly don't know how more easily such concept could be expressed.
IMHO at least "internal kernel handle" does not play look-alike games with
"file descriptor".



> 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)?

That'd mean placing an eventpoll custom hook into sys_close(). Looks very
bad to me, and probably will look even worse to other kernel folks.
Is not much a performance issue (a check to see if a file* is an eventpoll
file is as easy as comparing the f_op pointer), but a design/style issue.
On top of that, the interface is already out by many years, so changing it
will like going to cause problems.



- Davide




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