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SubjectRe: [patch 6/6] rtmutex: Avoid pointless requeueing in the deadlock detection chain walk
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On Wed, May 21, 2014 at 8:25 PM, Thomas Gleixner <tglx@linutronix.de> wrote:

> @@ -440,32 +452,41 @@ static int rt_mutex_adjust_prio_chain(st
> get_task_struct(task);
> raw_spin_lock_irqsave(&task->pi_lock, flags);
>
> - if (waiter == rt_mutex_top_waiter(lock)) {
> - /*
> - * The waiter became the top waiter on the
> - * lock. Remove the previous top waiter from the tasks
> - * pi waiters list and add waiter to it.
> - */
> - rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
> - rt_mutex_enqueue_pi(task, waiter);
> - __rt_mutex_adjust_prio(task);
> -
> - } else if (prerequeue_top_waiter == waiter) {
> - /*
> - * The waiter was the top waiter on the lock. Remove
> - * waiter from the tasks pi waiters list and add the
> - * new top waiter to it.
> - */
> - rt_mutex_dequeue_pi(task, waiter);
> - waiter = rt_mutex_top_waiter(lock);
> - rt_mutex_enqueue_pi(task, waiter);
> - __rt_mutex_adjust_prio(task);
> -
> - } else {
> - /*
> - * Nothing changed. No need to do any priority
> - * adjustment.
> - */
> + /*
> + * In case we are just following the lock chain for deadlock
> + * detection we can avoid the whole requeue and priority
> + * adjustment business.
> + */
> + if (requeue) {
> + if (waiter == rt_mutex_top_waiter(lock)) {
> + /*
> + * The waiter became the top waiter on the
> + * lock. Remove the previous top waiter from
> + * the tasks pi waiters list and add waiter to
> + * it.
> + */
> + rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
> + rt_mutex_enqueue_pi(task, waiter);
> + __rt_mutex_adjust_prio(task);
> +
> + } else if (prerequeue_top_waiter == waiter) {
> + /*
> + * The waiter was the top waiter on the
> + * lock. Remove waiter from the tasks pi
> + * waiters list and add the new top waiter to
> + * it.
> + */
> + rt_mutex_dequeue_pi(task, waiter);
> + waiter = rt_mutex_top_waiter(lock);
> + rt_mutex_enqueue_pi(task, waiter);
> + __rt_mutex_adjust_prio(task);
> +
> + } else {
> + /*
> + * Nothing changed. No need to do any priority
> + * adjustment.
> + */
> + }
> }
>
> raw_spin_unlock_irqrestore(&task->pi_lock, flags);

In the above case, could we go 1 step further and avoid taking the pi
lock as well?

if (requeue) {
raw_spin_lock_irqsave(&task->pi_lock, flags);

if (waiter == rt_mutex_top_waiter(lock)) {
/*
* The waiter became the top waiter on the
* lock. Remove the previous top waiter from
* the tasks pi waiters list and add waiter to
* it.
*/
rt_mutex_dequeue_pi(task, prerequeue_top_waiter);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);

} else if (prerequeue_top_waiter == waiter) {
/*
* The waiter was the top waiter on the
* lock. Remove waiter from the tasks pi
* waiters list and add the new top waiter to
* it.
*/
rt_mutex_dequeue_pi(task, waiter);
waiter = rt_mutex_top_waiter(lock);
rt_mutex_enqueue_pi(task, waiter);
__rt_mutex_adjust_prio(task);

} else {
/*
* Nothing changed. No need to do any priority
* adjustment.
*/
}

raw_spin_unlock_irqrestore(&task->pi_lock, flags);
}


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