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    SubjectRe: [PATCH] io stalls (was: -rc7 Re: Linux 2.4.21-rc6)
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    On Tue, 2003-06-10 at 20:33, Andrea Arcangeli wrote:
    > On Mon, Jun 09, 2003 at 05:39:23PM -0400, Chris Mason wrote:
    > > + if (!waitqueue_active(&q->wait_for_requests[rw]))
    > > + clear_queue_full(q, rw);
    >
    > you've an smp race above, the smp safe implementation is this:
    >

    clear_queue_full has a wmb() in my patch, and queue_full has a rmb(), I
    thought that covered these cases? I'd rather remove those though, since
    the spot you point out is the only place done outside the
    io_request_lock.

    > if (!waitqueue_active(&q->wait_for_requests[rw])) {
    > clear_queue_full(q, rw);
    > mb();
    > if (unlikely(waitqueue_active(&q->wait_for_requests[rw])))
    > wake_up(&q->wait_for_requests[rw]);
    > }
    >
    I don't think we need the extra wake_up (this is in __get_request_wait,
    right?), since it gets done by get_request_wait_wakeup()

    > I'm also unsure what the "waited" logic does, it doesn't seem necessary.

    Once a process waits once, they are allowed to ignore the q->full flag.
    This way existing waiters can make progress even when q->full is set.
    Without the waited check, q->full will never get cleared because the
    last writer wouldn't proceed until the last writer was gone. I had to
    make __get_request for the same reason.

    -chris


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