linux-kernel - Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic

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Date:	Fri, 12 Feb 2016 21:23:55 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()

On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> This patch adds a new waiter parameter to the mutex_optimistic_spin()
> function to prepare it to be used by a waiter-spinner that doesn't
> need to go into the OSQ as there can only be one waiter-spinner which
> is the head of the waiting queue.

Does not explain why..

>  static bool mutex_optimistic_spin(struct mutex *lock,
> +				  struct ww_acquire_ctx *ww_ctx,
> +				  const bool use_ww_ctx, int waiter)
>  {
>  	struct task_struct *task = current;
> +	bool acquired = false;
>  
> +	if (!waiter) {
> +		if (!mutex_can_spin_on_owner(lock))
> +			goto done;

Why doesn't the waiter have to check mutex_can_spin_on_owner() ?

>  
> +		/*
> +		 * In order to avoid a stampede of mutex spinners trying to
> +		 * acquire the mutex all at once, the spinners need to take a
> +		 * MCS (queued) lock first before spinning on the owner field.
> +		 */
> +		if (!osq_lock(&lock->osq))
> +			goto done;
> +	}
>  
>  	while (true) {
>  		struct task_struct *owner;