linux-kernel - Re: [patch 2/2] rtmutex: Avoid pointless requeueing in the deadlock detection chain walk

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Date:	Thu, 15 May 2014 17:39:37 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [patch 2/2] rtmutex: Avoid pointless requeueing in the deadlock
 detection chain walk

On Wed, 14 May 2014 20:03:27 -0000
Thomas Gleixner <tglx@...utronix.de> wrote:
 
>  	/* Release the task */
>  	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> +	/*
> +	 * We must abort the chain walk if there is no lock owner even
> +	 * in the dead lock detection case, as we have nothing to
> +	 * follow here.
> +	 */
>  	if (!rt_mutex_owner(lock)) {
>  		/*
>  		 * If the requeue above changed the top waiter, then we need
>  		 * to wake the new top waiter up to try to get the lock.
>  		 */
> -
>  		if (top_waiter != rt_mutex_top_waiter(lock))
>  			wake_up_process(rt_mutex_top_waiter(lock)->task);

Seems we can change the above if condition to:

	if (requeue && top_waiter != rt_mutex_top_waiter(lock))

as there was no "requeue above" that could change anything.

-- Steve


>  		raw_spin_unlock(&lock->wait_lock);
> @@ -404,18 +413,20 @@ 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)) {
> -		/* Boost the owner */
> -		rt_mutex_dequeue_pi(task, top_waiter);
> -		rt_mutex_enqueue_pi(task, waiter);
> -		__rt_mutex_adjust_prio(task);
> -
> -	} else if (top_waiter == waiter) {
> -		/* Deboost the owner */
> -		rt_mutex_dequeue_pi(task, waiter);
> -		waiter = rt_mutex_top_waiter(lock);
> -		rt_mutex_enqueue_pi(task, waiter);
> -		__rt_mutex_adjust_prio(task);
> +	if (requeue) {
> +		if (waiter == rt_mutex_top_waiter(lock)) {
> +			/* Boost the owner */
> +			rt_mutex_dequeue_pi(task, top_waiter);
> +			rt_mutex_enqueue_pi(task, waiter);
> +			__rt_mutex_adjust_prio(task);
> +
> +		} else if (top_waiter == waiter) {
> +			/* Deboost the owner */
> +			rt_mutex_dequeue_pi(task, waiter);
> +			waiter = rt_mutex_top_waiter(lock);
> +			rt_mutex_enqueue_pi(task, waiter);
> +			__rt_mutex_adjust_prio(task);
> +		}
>  	}
>  
>  	raw_spin_unlock_irqrestore(&task->pi_lock, flags);
> 

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