Subject: Re: [PATCH] net: fix race in the receive/select

From: Jarek Poplawski <[email-masked]>
Date: Sun, 28 Jun 2009 13:22:14 +0200

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

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