linux-kernel - Re: [PATCH 3/8] sched/wait: Use __add_wait_queue{_tail}

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Message-ID: <20140415134949.GU11096@twins.programming.kicks-ass.net>
Date:	Tue, 15 Apr 2014 15:49:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dongsheng Yang <yangds.fnst@...fujitsu.com>
Cc:	rostedt@...dmis.org, fweisbec@...il.com, mingo@...hat.com,
	acme@...stprotocols.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] sched/wait: Use __add_wait_queue{_tail}_exclusive()
 as possible.

On Tue, Apr 15, 2014 at 09:32:52PM +0900, Dongsheng Yang wrote:
> From: Dongsheng <yangds.fnst@...fujitsu.com>
> 
> There is already a function in include/linux/wait.h to cover the
> 'exclusive' usage. So we can use it in sched/wait.c to replace
> the opened implementation of it.
> 
> Signed-off-by: Dongsheng <yangds.fnst@...fujitsu.com>
> ---
>  kernel/sched/wait.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
> index 283750e..b04827e 100644
> --- a/kernel/sched/wait.c
> +++ b/kernel/sched/wait.c
> @@ -38,9 +38,8 @@ void add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t *wait)
>  	unsigned long flags;
>  
>  	trace_sched_wait(current);
> -	wait->flags |= WQ_FLAG_EXCLUSIVE;
>  	spin_lock_irqsave(&q->lock, flags);
> -	__add_wait_queue_tail(q, wait);
> +	__add_wait_queue_tail_exclusive(q, wait);
>  	spin_unlock_irqrestore(&q->lock, flags);
>  }
>  EXPORT_SYMBOL(add_wait_queue_exclusive);
> @@ -192,10 +191,9 @@ prepare_to_wait_exclusive(wait_queue_head_t *q, wait_queue_t *wait, int state)
>  	unsigned long flags;
>  
>  	trace_sched_wait(current);
> -	wait->flags |= WQ_FLAG_EXCLUSIVE;
>  	spin_lock_irqsave(&q->lock, flags);
>  	if (list_empty(&wait->task_list))
> -		__add_wait_queue_tail(q, wait);
> +		__add_wait_queue_tail_exclusive(q, wait);
>  	set_current_state(state);
>  	spin_unlock_irqrestore(&q->lock, flags);

That is not a no-op, if !list_empty() we loose the WQ_flag
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