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SubjectRe: epoll design problems with common fork/exec patterns
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).


> 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.

> 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.

- Davide

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