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

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Date:	Mon, 15 Feb 2016 18:15:30 -0800
From:	Jason Low <jason.low2@...com>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Waiman Long <Waiman.Long@....com>,
	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>,
	"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, 2016-02-12 at 14:14 -0800, Davidlohr Bueso wrote:
> On Fri, 12 Feb 2016, Peter Zijlstra wrote:
> 
> >On Fri, Feb 12, 2016 at 12:32:12PM -0500, Waiman Long wrote:
> >>  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() ?
> 
> afaict because mutex_can_spin_on_owner() fails immediately when the counter
> is -1, which is a nono for the waiters case.

mutex_can_spin_on_owner() returns false if the task needs to reschedule
or if the lock owner is not on_cpu. In either case, the task will end up
not spinning when it enters the spin loop. So it makes sense if the
waiter also checks mutex_can_spin_on_owner() so that the optimistic spin
queue overhead can be avoided in those cases.

Jason