linux-kernel - Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to mutex_optimistic

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Date:	Fri, 12 Feb 2016 21:40:27 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ding Tianhong <dingtianhong@...wei.com>,
	Jason Low <jason.low2@....com>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()

On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> @@ -358,8 +373,8 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>  			}
>  
>  			mutex_set_owner(lock);
> -			osq_unlock(&lock->osq);
> -			return true;
> +			acquired = true;
> +			break;
>  		}
>  
>  		/*
> @@ -380,7 +395,10 @@ static bool mutex_optimistic_spin(struct mutex *lock,
>  		cpu_relax_lowlatency();
>  	}
>  
> -	osq_unlock(&lock->osq);
> +	if (!waiter)
> +		osq_unlock(&lock->osq);
> +	if (acquired || waiter)
> +		return acquired;
>  done:
>  	/*
>  	 * If we fell out of the spin path because of need_resched(),

Is there a reason to not also preempt in the wait-loop? Surely the same
reason is still valid there too?