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    SubjectRe: epoll design problems with common fork/exec patterns
    On Sat, 27 Oct 2007, Willy Tarreau wrote:

    > On Sat, Oct 27, 2007 at 09:59:07AM -0700, Davide Libenzi wrote:
    > > On Sat, 27 Oct 2007, Marc Lehmann wrote:
    > >
    > > > > Please provide some code to illustrate one exact problem you have.
    > > >
    > > > // assume there is an open epoll set that listens for events on fd 5
    > > > if (fork () = 0)
    > > > {
    > > > close (5);
    > > > // fd 5 is now removed from the epoll set of the parent.
    > > > _exit (0);
    > > > }
    > >
    > > Hmmm ... what? I assume you know that:
    > >
    > > 1) A file descriptor is a userspace view/handle of a kernel object
    > >
    > > 2) The kernel object has a use-count for as many file descriptors that
    > > have been handed out to userspace
    > >
    > > 3) A close() decreases the internal counter by one
    > >
    > > 4) The kernel object gets effectively closed when the internal counter
    > > goes to zero
    > >
    > > 5) A fork() acts as a dup() on the file descriptors by hence bumping up
    > > its internal counter
    > >
    > > 6) Epoll removes the file from the set, when the *kernel* object gets
    > > closed (internal use-count goes to zero)
    > >
    > > With that in mind, how can the code snippet above trigger a removal from
    > > the epoll set?
    >
    > Davide,
    >
    > from what I understand, Marc is not asking for the code above to remove
    > the fd from the epoll set, but he's in fact complaining that he *observed*
    > that the fd was removed from the epoll set in the *parent* process when
    > the child closes it, which is of course not expected at all. As strange
    > as it looks like, this might need investigation. It is possible that there
    > is some strange bug somewhere in some kernel versions.

    That would be *really* strange, since epoll hooks in __fput() in order to
    perform proper cleanup. This means that, in the case above, the file will
    be really closed in the parent too. That, I think, would trigger way more
    serious problems in userspace.



    > Marc, I think that if you indicate the last kernel version on which you
    > observed this and provide a very short and easy reproducer, it would
    > help everyone investigating this. Basically something which reports "OK"
    > or "KO".

    Of course. That'd be great.



    - Davide


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