Re: [PATCH 2/2] sched/wait: avoid abort_exclusive_wait() in __wait_on_bit_lock()

[Oleg Nesterov <oleg@redhat.com>]

On 08/26, Oleg Nesterov wrote:
>
> We do not need anything tricky to avoid the race, we can just call
> finish_wait() if action() fails. test_and_set_bit() implies mb() so
> the lockless list_empty_careful() case is fine, we can not miss the
> condition if we race with unlock_page().

To simplify the review see the code with the patch applied:


int __sched
__wait_on_bit_lock(wait_queue_head_t *wq, struct wait_bit_queue *q,
			wait_bit_action_f *action, unsigned mode)
{
	int ret = 0;

	for (;;) {
		prepare_to_wait_exclusive(wq, &q->wait, mode);
		if (test_bit(q->key.bit_nr, q->key.flags)) {
			ret = action(&q->key, mode);
			/*
			 * Ensure that clear_bit() + wake_up() right after
			 * test_and_set_bit() below can't see us; it should
			 * wake up another exclusive waiter if we fail.
			 */
			if (ret)
				finish_wait(wq, &q->wait);
		}
		if (!test_and_set_bit(q->key.bit_nr, q->key.flags)) {
			if (!ret)
				finish_wait(wq, &q->wait);
			return 0;
		} else if (ret) {
			return ret;
		}
	}
}