Re: net: Generalise wq_has_sleeper helper

[David Miller]

From: Peter Zijlstra 
Date: Wed, 25 Nov 2015 10:15:33 +0100

> On Tue, Nov 24, 2015 at 01:54:23PM +0800, Herbert Xu wrote:
>> + * The race for tcp fires when the __add_wait_queue changes done by CPU1 
>> stay
>> + * in its cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1
>> + * could then endup calling schedule and sleep forever if there are no more
>> + * data on the socket.
>> + *
> 
> Would be easier to refer to the comment that now adorns
> waitqueue_active().

Yeah, that might be a good idea.  Herbert can you adjust this?

>> + */
>> +static inline bool wq_has_sleeper(wait_queue_head_t *wq)
>> +{
>> +/* We need to be sure we are in sync with the
> 
> broken comment style.

This is how we do it in the networking, so that's why it's formatted
this way, but yes he will need to fix it up.
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Re: net: Generalise wq_has_sleeper helper

[Peter Zijlstra]

On Tue, Nov 24, 2015 at 01:54:23PM +0800, Herbert Xu wrote:
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index 1e1bf9f..bd1157f 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -107,6 +107,50 @@ static inline int waitqueue_active(wait_queue_head_t *q)
>   return !list_empty(>task_list);
>  }
>  
> +/**
> + * wq_has_sleeper - check if there are any waiting processes
> + * @wq: wait queue head
> + *
> + * Returns true if wq has waiting processes
> + *
> + * The purpose of the wq_has_sleeper and sock_poll_wait is to wrap the memory
> + * barrier call. They were added due to the race found within the tcp code.
> + *
> + * Consider following tcp code paths:
> + *
> + * CPU1  CPU2
> + *
> + * sys_selectreceive packet
> + *   ... ...
> + *   __add_wait_queueupdate tp->rcv_nxt
> + *   ... ...
> + *   tp->rcv_nxt check   sock_def_readable
> + *   ... {
> + *   schedule   rcu_read_lock();
> + *  wq = rcu_dereference(sk->sk_wq);
> + *  if (wq && waitqueue_active(>wait))
> + *  wake_up_interruptible(>wait)
> + *  ...
> + *   }
> + *
> + * The race for tcp fires when the __add_wait_queue changes done by CPU1 stay
> + * in its cache, and so does the tp->rcv_nxt update on CPU2 side.  The CPU1
> + * could then endup calling schedule and sleep forever if there are no more
> + * data on the socket.
> + *

Would be easier to refer to the comment that now adorns
waitqueue_active().

> + */
> +static inline bool wq_has_sleeper(wait_queue_head_t *wq)
> +{
> + /* We need to be sure we are in sync with the

broken comment style.

> +  * add_wait_queue modifications to the wait queue.
> +  *
> +  * This memory barrier should be paired with one on the
> +  * waiting side.
> +  */
> + smp_mb();
> + return waitqueue_active(wq);
> +}
> +
>  extern void add_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
>  extern void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t 
> *wait);
>  extern void remove_wait_queue(wait_queue_head_t *q, wait_queue_t *wait);
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Re: net: Generalise wq_has_sleeper helper

[Herbert Xu]

On Tue, Nov 24, 2015 at 04:30:25PM -0500, David Miller wrote:
>
> I'm fine with wherever this patch goes.  Herbert is there any
> particular tree where it'll facilitate another user quickest?
> 
> Or should I just toss it into net-next?
> 
> Acked-by: David S. Miller 

No Dave net-next is fine I think.  This was prompted by Tatsukawa-san's
patches to fix waitqueue users affected by this very race and they
were all over the tree.

Thanks,
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Email: Herbert Xu 
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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Re: net: Generalise wq_has_sleeper helper

[David Miller]

From: Herbert Xu 
Date: Tue, 24 Nov 2015 13:54:23 +0800

> On Wed, Nov 11, 2015 at 05:48:29PM +0800, Herbert Xu wrote:
>>
>> BTW, the networking folks found this years ago and even added
>> helpers to deal with this.  See for example wq_has_sleeper in
>> include/net/sock.h.  It would be good if we can move some of
>> those helpers into wait.h instead.
> 
> Here is a patch against net-next which makes the wq_has_sleeper
> helper available to non-next users:
> 
> ---8<---
> The memory barrier in the helper wq_has_sleeper is needed by just
> about every user of waitqueue_active.  This patch generalises it
> by making it take a wait_queue_head_t directly.  The existing
> helper is renamed to skwq_has_sleeper.
> 
> Signed-off-by: Herbert Xu 

I'm fine with wherever this patch goes.  Herbert is there any
particular tree where it'll facilitate another user quickest?

Or should I just toss it into net-next?

Acked-by: David S. Miller 
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