linux-kernel - Re: [PATCH v3 1/4] wait.[ch]: Introduce the simple waitqueue (swait) implementation

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Message-ID: <20151026141949.GA26372@fixme-laptop.cn.ibm.com>
Date:	Mon, 26 Oct 2015 22:19:49 +0800
From:	Boqun Feng <boqun.feng@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Daniel Wagner <daniel.wagner@...-carit.de>,
	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 1/4] wait.[ch]: Introduce the simple waitqueue (swait)
 implementation

On Mon, Oct 26, 2015 at 02:26:01PM +0100, Peter Zijlstra wrote:
> On Mon, Oct 26, 2015 at 01:59:44PM +0100, Daniel Wagner wrote:
> > Hi Boqun,
> > 
> > On 10/26/2015 01:04 PM, Boqun Feng wrote:
> > > On Tue, Oct 20, 2015 at 09:28:07AM +0200, Daniel Wagner wrote:
> > >> +
> > >> +/*
> > >> + * The thing about the wake_up_state() return value; I think we can ignore it.
> > >> + *
> > >> + * 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)
> > >> +{
> > >> +	struct swait_queue *curr;
> > >> +
> > >> +	list_for_each_entry(curr, &q->task_list, task_list) {
> > >> +		wake_up_process(curr->task);
> > >> +		list_del_init(&curr->task_list);
> > >> +		break;
> > > 
> > > Just be curious, what's this break for? Or what's this loop(?) for?
> > 
> > I have to guess here, since Peter wrote it. It looks like the function
> > is based on __wake_up_common(). Though I agree the loop is not necessary
> > and something like below should the trick. Unless I do not see something
> > important.
> > 
> > 	void swake_up_locked(struct swait_queue_head *q)
> > 	{
> > 		struct swait_queue *curr;
> > 
> > 		if (list_emtpy(&q))
> > 			return;
> > 
> > 		curr = list_first_entry(&q, typeof(*curr), task_list);
> > 		wake_up_process(curr->task);
> > 		list_del_init(&curr->task_list);
> > 	}
> > 
> > If Peter is not complaining I change swake_up_locked() for the next version.

This gains better readability, I think ;-)

> 
> Yes, that is equivalent, just more code. As I wrote in my last email; I
> was lazy :-)

;-)

Maybe introduce a list_pick_one_if_any() macro for convenience:

	#define list_pick_one_if_any(pos, list, member) 	\
	if (!list_empty(list) && (pos = list_first_entry(list, typeof(*pos), member), 1)) 

then

	void swake_up_locked(struct swait_queue_head *q)
	{
		struct swait_queue *curr;

		list_pick_one_if_any(curr, q->task_list, task_list) {
			wake_up_process(curr->task);
			list_del_init(&curr->task_list);
		}
	}
	


Anyway, thank you both for going through this.

Regards,
Boqun

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