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

[Davidlohr Bueso]

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.

Thanks,
Davidlohr