On 06/25, Davide Libenzi wrote:

Not all the code that uses add_wait_queue() does need to have the MB,
like code that does the most common pattern:

xxx_poll(...) {
	poll_wait(...);
	lock();
	flags = calc_flags(->status);
	unlock();
	return flags;
}

xxx_update(...) {
	lock();
	->status = ...;
	unlock();
	if (waitqueue_active())
		wake_up();
}

It's the code that does the lockless flags calculation in ->poll that
might need it.

And if we remove waitqueue_active() in xxx_update(), then lock/unlock is
not needed too.

If xxx_poll() takes q->lock first, it can safely miss the changes in ->status
and schedule(): xxx_update() will take q->lock, notice the sleeper and wake
it up (ok, it will set ->triggered but this doesn't matter).

If xxx_update() takes q->lock first, xxx_poll() must see the changes in
status after poll_wait()->unlock(&q->lock) (in fact, after lock, not unlock).

Oleg.