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;
