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    SubjectRe: Unexpected cascaded epoll behavior - my mistake or kernel bug
    On Fri, 23 Jan 2009, Pavel Pisa wrote:

    > Hello Davide and all others,
    >
    > I have got to implementing yet another event library and I experience
    > something strange when level triggered epoll_wait() monitors another
    > level triggered epoll set. Top level epoll_wait() return information
    > about one event pending. The event points correctly to the monitored
    > second level epoll fd, but epoll_wait() on this fd with 0 or small timeout
    > returns zero/no events and top level epoll reporting is not reset at this
    > point so program enters busy loop. If there are events, they are processed
    > correctly.

    I'm ashamed to say, I know. It has been reported to me about three months
    ago, I coded patches, but I did not have the time to test them carefully.
    The problem is that inside the epoll wakeup callback, we don't know which
    even we got (and we can't call ->poll()). So we add the fd to the
    ready-list and we wake up the waiters. The current code simply does return
    EPOLLIN (from epoll's poll()) if the ready-list is not empty. The problem
    is that the even we received in the epoll callback could have been a
    POLLIN when the fd is waiting for POLLOUT. THis situations gets sorted out
    in epoll_wait(), but inside epoll's poll() only a quick test was used
    (ready-list not empty).
    The patch was not exactly a one-liner, so I need a few days to double
    check it and test it. I'll try to do it this w/end.



    - Davide




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