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

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Message-ID: <20160213121052.GB6357@twins.programming.kicks-ass.net>
Date:	Sat, 13 Feb 2016 13:10:52 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Davidlohr Bueso <dave@...olabs.net>
Cc:	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, Feb 12, 2016 at 02:14:44PM -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.

Can't see it do that, also, if it were to do that, we'd not be here
since having a waiter would then mean no spinners and no starvation
etc..