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Message-ID: <1455588930.2276.36.camel@j-VirtualBox>
Subject: Re: [PATCH v2 1/4] locking/mutex: Add waiter parameter to
 mutex_optimistic_spin()
From: Jason Low <jason.low2@hp.com>
To: Davidlohr Bueso <dave@stgolabs.net>
Cc: Peter Zijlstra <peterz@infradead.org>, 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>
Date: Mon, 15 Feb 2016 18:15:30 -0800
In-Reply-To: <20160212221444.GC16417@linux-uzut.site>
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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