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    Subjectfutex: wakeup race and futex_q woken state definition
    I'm working on futex commentary cleanup patch series.  While reading
    through all the remaining comments, I've come across a couple I'd your
    thoughts on:

    The futex woken state is defined as:

    * A futex_q has a woken state, just like tasks have TASK_RUNNING.
    * It is considered woken when plist_node_empty(&q->list) || q->lock_ptr == 0.
    * The order of wakup is always to make the first condition true, then
    * wake up q->waiter, then make the second condition true.

    1) wake_futex() actually wakes the task (q->task not q->waiter) after
    the lock_ptr has been set to NULL. I believe this is fine and can
    correct the comments accordingly.

    2) futex_wait_queue_me() (recently refactored from futex_wait())
    performs the following test:

    * !plist_node_empty() is safe here without any lock.
    * q.lock_ptr != 0 is not safe, because of ordering against wakeup.
    if (likely(!plist_node_empty(&q->list))) {
    * If the timer has already expired, current will already be
    * flagged for rescheduling. Only call schedule if there
    * is no timeout, or if it has yet to expire.
    if (!timeout || timeout->task)

    As I understand it, this is to avoid a missed wakeup when a FUTEX_WAKE
    call occurs after the queue_me() but before the futex_wait() call has
    had a chance to call schedule() (via futex_wait_queue_me()). However,
    as no locks are taken, I don't see what prevents the futex_q from being
    removed from the hash list after the plist_node_empty() test and before
    the call to schedule(). In this scenario, the futex_q will not be found
    on the hash list by subsequent wakers, and it will remain in schedule()
    until a timeout or signal occurs.

    This leads me to the question on the comment: "!plist_node_empty() is
    safe here without any lock." - Why is that safe?

    Secondly, why is the q.lock_ptr test not safe? "q.lock_ptr != 0 is not
    safe, because of ordering against wakeup."

    I understand the definition of the woken state to be
    "plist_node_empty(&q->list) || q->lock_ptr == 0". So testing the plist
    will detect a woken futex sooner than testing for a null lock_ptr, but I
    don't see how one is more "safe" than the other when no locks are held
    to prevent the futex_q from vanishing off the list before the call to


    Darren Hart
    IBM Linux Technology Center
    Real-Time Linux Team

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      Last update: 2009-09-17 01:53    [W:0.026 / U:0.656 seconds]
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