Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic_spin()

[Peter Zijlstra <peterz@infradead.org>]

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;