Re: [RFC v4] wait: prevent waiter starvation in __wait_on_bit_lock

[Oleg Nesterov <oleg@redhat.com>]

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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