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Date:   Thu, 9 Nov 2017 11:06:53 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>, kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched/swait: allow swake_up() to return

On 09/11/2017 10:18, Peter Xu wrote:
> Let swake_up() to return whether any of the waiters is waked up. One use
> case of it would be:
> 
>   if (swait_active(wq)) {
>     swake_up(wq);
>     // do something when waiter is waked up
>     waked_up++;
>   }
> 
> Logically it's possible that when reaching swake_up() the wait queue is
> not active any more, and here doing something like waked_up++ would be
> inaccurate.  To correct it, we need an atomic version of it.
> 
> With this patch, we can simply re-write it into:
> 
>   if (swake_up(wq)) {
>     // do something when waiter is waked up
>     waked_up++;
>   }
> 
> After all we are checking swait_active() inside swake_up() too.

Better subject:

sched/swait: make swake_up() return whether there were any waiters

I like this patch.

> 
> CC: Ingo Molnar <mingo@...hat.com>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Peter Xu <peterx@...hat.com>
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---
>  include/linux/swait.h |  4 ++--
>  kernel/sched/swait.c  | 21 ++++++++++++++++-----
>  2 files changed, 18 insertions(+), 7 deletions(-)
> 
> diff --git a/include/linux/swait.h b/include/linux/swait.h
> index c1f9c62a8a50..c55171280ac8 100644
> --- a/include/linux/swait.h
> +++ b/include/linux/swait.h
> @@ -84,9 +84,9 @@ static inline int swait_active(struct swait_queue_head *q)
>  	return !list_empty(&q->task_list);
>  }
>  
> -extern void swake_up(struct swait_queue_head *q);
> +extern bool swake_up(struct swait_queue_head *q);
>  extern void swake_up_all(struct swait_queue_head *q);
> -extern void swake_up_locked(struct swait_queue_head *q);
> +extern bool swake_up_locked(struct swait_queue_head *q);
>  
>  extern void __prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait);
>  extern void prepare_to_swait(struct swait_queue_head *q, struct swait_queue *wait, int state);
> diff --git a/kernel/sched/swait.c b/kernel/sched/swait.c
> index 3d5610dcce11..31f8d677c690 100644
> --- a/kernel/sched/swait.c
> +++ b/kernel/sched/swait.c
> @@ -16,29 +16,40 @@ EXPORT_SYMBOL(__init_swait_queue_head);
>   * If for some reason it would return 0, that means the previously waiting
>   * task is already running, so it will observe condition true (or has already).
>   */
> -void swake_up_locked(struct swait_queue_head *q)
> +bool swake_up_locked(struct swait_queue_head *q)
>  {
>  	struct swait_queue *curr;
>  
>  	if (list_empty(&q->task_list))
> -		return;
> +		return false;
>  
>  	curr = list_first_entry(&q->task_list, typeof(*curr), task_list);
>  	wake_up_process(curr->task);
>  	list_del_init(&curr->task_list);
> +	return true;
>  }
>  EXPORT_SYMBOL(swake_up_locked);
>  
> -void swake_up(struct swait_queue_head *q)
> +/**
> + * swake_up - Wake up one process on the waiting list
> + * @q:        the waitqueue to wake up
> + *
> + * Returns true if some process is waked up, otherwise false if there
> + * is no waiter to wake up.
> + */
> +bool swake_up(struct swait_queue_head *q)
>  {
> +	bool ret;
>  	unsigned long flags;
>  
>  	if (!swait_active(q))
> -		return;
> +		return false;
>  
>  	raw_spin_lock_irqsave(&q->lock, flags);
> -	swake_up_locked(q);
> +	ret = swake_up_locked(q);
>  	raw_spin_unlock_irqrestore(&q->lock, flags);
> +
> +	return ret;
>  }
>  EXPORT_SYMBOL(swake_up);
>  
> 

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