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    SubjectRe: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock
    On 01/21, Johannes Weiner wrote:
    >
    > @@ -187,6 +187,31 @@ __wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
    > }
    > } while (test_and_set_bit(q->key.bit_nr, q->key.flags));
    > finish_wait(wq, &q->wait);
    > + if (unlikely(ret)) {
    > + /*
    > + * Contenders are woken exclusively. If we were woken
    > + * by an unlock we have to take the lock ourselves and
    > + * wake the next contender on unlock. But the waiting
    > + * function failed, we do not take the lock and won't
    > + * unlock in the future. Make sure the next contender
    > + * does not wait forever on an unlocked bit.
    > + *
    > + * We can also get here without being woken through
    > + * the waitqueue, so there is a small chance of doing a
    > + * bogus wake up between an unlock clearing the bit and
    > + * the next contender being woken up and setting it again.
    > + *
    > + * It does no harm, though, the scheduler will ignore it
    > + * as the process in question is already running.
    > + *
    > + * The unlock path clears the bit and then wakes up the
    > + * next contender. If the next contender is us, the
    > + * barrier makes sure we also see the bit cleared.
    > + */
    > + smp_rmb();
    > + if (!test_bit(q->key.bit_nr, q->key.flags)))
    > + __wake_up_bit(wq, q->key.flags, q->key.bit_nr);

    I think this is correct, and (unfortunately ;) you are right:
    we need rmb() even after finish_wait().

    And we have to check ret twice, and the false wakeup is still
    possible. This is minor, but just for discussion, can't we do
    this differently?

    int finish_wait_xxx(wait_queue_head_t *q, wait_queue_t *wait)
    {
    unsigned long flags;
    int woken;

    __set_current_state(TASK_RUNNING);
    spin_lock_irqsave(&q->lock, flags);
    woken = list_empty(&wait->task_list);
    list_del_init(&wait->task_list);
    spin_unlock_irqrestore(&q->lock, flags);

    return woken;
    }

    Now, __wait_on_bit_lock() does:

    if (test_bit(q->key.bit_nr, q->key.flags)) {
    if ((ret = (*action)(q->key.flags))) {
    if (finish_wait_xxx(...))
    __wake_up_bit(...);
    return ret;
    }
    }

    Or we can introduce

    int finish_wait_yyy(wait_queue_head_t *q, wait_queue_t *wait,
    int mode, void *key)
    {
    unsigned long flags;
    int woken;

    __set_current_state(TASK_RUNNING);
    spin_lock_irqsave(&q->lock, flags);
    woken = list_empty(&wait->task_list);
    if (woken)
    __wake_up_common(q, mode, 1, key);
    else
    list_del_init(&wait->task_list);
    spin_unlock_irqrestore(&q->lock, flags);

    return woken;
    }

    Perhaps a bit too much for this particular case, but I am thinking
    about other cases when we need to abort the exclusive wait.

    For example, don't we have the similar problems with
    wait_event_interruptible_exclusive() ?

    Oleg.



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