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

[Waiman Long]

On 02/12/2016 03:23 PM, Peter Zijlstra wrote:
> 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() ?

The reason why regular spinner need to call mutex_can_spin_on_owner is 
because we want to avoid the overhead of the OSQ if  we can't spin. As 
mutex_can_spin_on_owner() is similar to mutex_spin_on_owner(), the 
waiter-spinner will exit spinning after calling mutex_spin_on_owner().

Cheers,
Longman