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    SubjectRE: PROBLEM: I/O system call never returns if file desc is closed in the
    Date

    > At 22:35 +0100 2001-06-06, Alan Cox wrote:

    > > > This report describes a problem in the usage of file
    > > > descriptors across

    > >> multiple threads. When one thread closes a file descriptor, another
    > >> thread which waits for an I/O on that file descriptor is not notified
    > >> and blocks forever.

    > >THe I/O does not block forever, it blocks until completed.

    > That's still "forever" if you don't specify a timeout in the select.

    If you don't want to block until an operation completes, then don't ask to!

    > >The actual final
    > >closure of the object occurs when the last operation on it exits

    > Select is defined as to return, with the appropriate bit set, if/when
    > a nonblocking read/write on the file descriptor won't block. You'd
    > get EBADF in this case, therefore causing the select to return would
    > be a Good Thing.

    That is not quite correct. That is a good approximate definition of
    'select's behavior, but it is not exact. As for your assertion that a
    noblocking read/write wouldn't block, that's not necessarily true. Remember,
    the 'close' may not have taken affect yet, since the descriptor is still in
    use by virtue of being selected on.

    > A related problem is that the second thread my be inside a blocking
    > read() instead of a select() call. It'd never continue. :-(

    Perfect. Doing this is absolutely, positively wrong and the more you are
    punished for it, the better. It's as wrong as calling 'free' on a chunk of
    memory when another thread may be usign it. It is impossible to make this
    work safely, as another thread could open a socket or file and get the same
    descriptor write before the call to 'read' is entered. There's no possible
    way to do this because there is no 'unlock a mutex and read' operation.

    > HOWEVER: IMHO it's bad design to distribute the responsibility for
    > file descriptors between threads.

    Why? That's a great design and it's absolutely essential in many cases.
    Suppose, for example, I have two descriptors I want to write to. If I assign
    one thread to each socket permanently, then I'm 100% guaranteed a context
    switch every time I change which socket I'm writing to, so if there's lots
    of small bits of data going out both of them, my performance will suck. But
    if I assign one thread to both socket descriptors, I'm guaranteed that one
    connection will stall if the the application-level send queue for the other
    has been swapped out to disk. Not distributing network I/O across threads
    dynamically is a recipe for either low performance or bursty performance.

    DS

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