Re: epoll design problems with common fork/exec patterns

Chris "ク" Heath
2008-02-27T01:40:10Z

On Tue, 2008-02-26 at 10:51 -0800, Davide Libenzi wrote:
> On Tue, 26 Feb 2008, Michael Kerrisk wrote:
>
> > Davide Libenzi wrote:
> > > On Sun, 28 Oct 2007, David Schwartz wrote:
> > >
> > >> Eric Dumazet wrote:
> > >>
> > >>> Events are not necessarly reported "by descriptors". epoll uses an opaque
> > >>> field provided by the user.
> > >>>
> > >>> It's up to the user to properly chose a tag that will makes sense
> > >>> if the user
> > >>> app is playing dup()/close() games for example.
> > >> Great. So the only issue then is that the documentation is confusing. It
> > >> frequently uses the term "fd" where it means file. For example, it says:
> > >>
> > >> Q1 What happens if you add the same fd to an
> > >> epoll_set
> > >> twice?
> > >>
> > >> A1 You will probably get EEXIST. However, it is
> > >> possible
> > >> that two threads may add the same fd twice. This is
> > >> a
> > >> harmless condition.
> > >>
> > >> This gives no reason to think there's anything wrong with adding the same
> > >> file twice so long as you do so through different descriptors. (One can
> > >> imagine an application that does this to segregate read and write operations
> > >> to avoid a race where the descriptor is closed from under a writer due to
> > >> handling a fatal read error.) Obviously, that won't work.
> > >
> > > I agree, that is confusing. However, you can safely add two different file
> > > descriptors pointing to the same file*, with different event masks, and
> > > that will work as expected.
> >
> > So can I summarize what I understand:
> >
> > a) Adding the same file descriptor twice to an epoll set will cause an
> > error (EEXIST).
>
> Yes.
>
>
>
> > b) In a separate message to linux-man, Chris Heath says that two threads
> > *can't* add the same fd twice to an epoll set, despite what the existing
> > man page text says. I haven't tested that, but it sounds to me as though
> > it is likely to be true. Can you comment please Davide?
>
> 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.

> > 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.

Chris