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Date: Sat, 13 Feb 2016 13:10:52 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Waiman Long <Waiman.Long@hpe.com>, Ingo Molnar <mingo@redhat.com>,
        linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Jason Low <jason.low2@hpe.com>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()
Message-ID: <20160213121052.GB6357@twins.programming.kicks-ass.net>
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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..