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Date: Fri, 12 Feb 2016 14:14:44 -0800
From: Davidlohr Bueso <dave@stgolabs.net>
To: Peter Zijlstra <peterz@infradead.org>
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: <20160212221444.GC16417@linux-uzut.site>
References: <1455298335-53229-1-git-send-email-Waiman.Long@hpe.com>
 <1455298335-53229-2-git-send-email-Waiman.Long@hpe.com>
 <20160212202355.GY6357@twins.programming.kicks-ass.net>
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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.

Thanks,
Davidlohr