Re: epoll design problems with common fork/exec patterns

[Michael Kerrisk]

On Wed, Feb 27, 2008 at 8:35 PM, Davide Libenzi <davidel@xxxxxxxxxxxxxxx> wrote:
> On Tue, 26 Feb 2008, Chris "ã~B¯" Heath wrote:
>
>  > On Tue, 2008-02-26 at 10:51 -0800, Davide Libenzi wrote:
>  > >
>
> > > Yes, you can't add the same fd twice. Think about a DB where "file*,fd" is
>  > > the key.
>  >
>  > To clarify, the key appears to be file* plus the user-space integer that
>  > represents the fd.
>
>  Yes, that's what I said.
>
>  > > > c) It is possible to add duplicated file descriptors referring to the same
>  > > > underlying open file description ("file *").  As you note, this can be a
>  > > > useful filtering technique, if the two file descriptors specify different
>  > > > masks.
>  > > >
>  > > > Assuming that is all correct, for man-pages-2.79, I've reworked the text
>  > > > for Q1/A1 as follows:
>  > > >
>  > > >        Q1     What  happens  if you add the same file descriptor
>  > > >               to an epoll set twice?
>  > > >
>  > > >        A1     You will probably get EEXIST.  However, it is pos-
>  > > >               sible   to   add  a  duplicate  (dup(2),  dup2(2),
>  > > >               fcntl(2) F_DUPFD, fork(2)) descriptor to the  same
>  > > >               epoll  set.   This  can  be a useful technique for
>  > > >               filtering events, if the duplicate  file  descrip-
>  > > >               tors are registered with different events masks.
>  > > >
>  > > > Seem okay Davide?
>  > >
>  > > Looks sane to me.
>  >
>  > I think fork(2) should not be in the above list.  fork(2) duplicates the
>  > kernel's fd, but the user-space integer that represents the fd remains
>  > the same, so you will get EEXIST if you try to add the fd that was
>  > duplicated by fork.
>
>  Good catch, fork(2) should not be there.

Okay -- removed.

But it is an ugly inconsistency.  On the one hand, a child process
cannot add the duplicate file descriptor to the epoll set.  (In every
other case that I can think of , descriptors duplicated by fork have
similar semantics to descriptors duplicated by dup() and friends.)  On
the other hand, the very fact that the child has a duplicate of the
descriptor means that even if the parent closes its descriptor, then
epoll_wait() in the parent will continue to receive notifications for
that descriptor because of the duplicated descriptor in the child.

The choice of [file *, fd] as the key for epoll sets really does seem
unfortunate.  Keying on [pid, fd] would have given saner semantics, it
seems to me.  Obviously it can't be changed now though.

Cheers,

Michael


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