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

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Date:	Mon, 15 Feb 2016 19:00:54 -0800
From:	Jason Low <jason.low2@...com>
To:	Waiman Long <waiman.long@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>, jason.low2@...com
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()

On Mon, 2016-02-15 at 18:55 -0500, Waiman Long wrote:
> On 02/12/2016 03:40 PM, Peter Zijlstra wrote:
> > 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?
> 
> The waiter does check for need_sched(). So it will break out of the loop 
> and return false in this case. This causes the waiter to loop back and 
> goes to sleep if the lock can't be acquired. That is why I don't think 
> we need to do another schedule_preempt_disabled() here.

The purpose of the additional reschedule point is to avoid delaying
preemption, which still applies if the spinner is a waiter. If it is a
waiter, the difference is that the delay isn't as long since it doesn't
need to be added to the wait_list. Nonetheless, preemption delays can
still occur, so I think the additional preemption point should also be
there in the waiter case.