Jarek Poplawski wrote, On 06/28/2009 01:10 PM:

Oleg Nesterov wrote, On 06/26/2009 04:50 PM:

quoted

On 06/26, Davide Libenzi wrote:

quoted

On Fri, 26 Jun 2009, Oleg Nesterov wrote:

quoted

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).

Sure. The snippet above was just to show what typically the code does, not
a suggestion on how to solve the socket case.

Yes, yes. I just meant you are right imho, we shouldn't add mb() into
add_wait_queue().

quoted

But yeah, the problem in this case is the waitqueue_active() call. Without
that, the wait queue lock/unlock in poll_wait() and the one in wake_up()
guarantees the necessary barriers.
Some might argue the costs of the lock/unlock of q->lock, and wonder if
MBs are a more efficient solution. This is something I'm not going into.
To me, it just looked not right having cross-matching MB in different
subsystems.

This is subjective and thus up to maintainers, but personally I think you
are very, very right.

Perhaps we can add

	void sock_poll_wait(struct file *file, struct sock *sk, poll_table *pt)
	{
		if (pt) {
			poll_wait(file, sk->sk_sleep, pt);
			/*
			 * fat comment
			 */
			smp_mb(); // or smp_mb__after_unlock();
		}
	}

Oleg.

Maybe 'a bit' further?:

static inline void __poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
	p->qproc(filp, wait_address, p);
}

static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
	if (p && wait_address)
		__poll_wait(filp, wait_address, p);
}

static inline void sock_poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
	if (p && wait_address) {
		__poll_wait(filp, wait_address, p);
		/*
		 * fat comment
		 */
		smp_mb(); // or smp_mb__after_unlock();
	}
}

Hmm... of course:

static inline void sock_poll_wait(struct file * filp, struct sock *sk, poll_table *p)
{
 	if (p && sk->sk_sleep) {
 		__poll_wait(filp, sk->sk_sleep, p);
 		/*
 		 * fat comment
 		 */
 		smp_mb(); // or smp_mb__after_unlock();
 	}
}
 

Jarek P.