Jarek Poplawski wrote, On 06/28/2009 01:10 PM:
> Oleg Nesterov wrote, On 06/26/2009 04:50 PM:

quoted text
>

>> On 06/26, Davide Libenzi wrote:

>>> On Fri, 26 Jun 2009, Oleg Nesterov wrote:

>>>

>>>> 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().

>>

>>> 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.
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