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

[Peter Zijlstra]

On Fri, Feb 12, 2016 at 02:14:44PM -0800, Davidlohr Bueso wrote:
> On Fri, 12 Feb 2016, Peter Zijlstra wrote:
> 
> >On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> >> 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() ?
> 
> afaict because mutex_can_spin_on_owner() fails immediately when the counter
> is -1, which is a nono for the waiters case.

Can't see it do that, also, if it were to do that, we'd not be here
since having a waiter would then mean no spinners and no starvation
etc..