Message-ID: <56FD8B3E.3000306@hpe.com>
Date: Thu, 31 Mar 2016 16:40:30 -0400
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
CC: 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>, Davidlohr Bueso <dave@stgolabs.net>,
        "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>
Subject: Re: [PATCH v3 2/3] locking/mutex: Enable optimistic spinning of woken
 task in wait queue
References: <1458668804-10138-1-git-send-email-Waiman.Long@hpe.com> <1458668804-10138-3-git-send-email-Waiman.Long@hpe.com> <20160329153935.GL3408@twins.programming.kicks-ass.net> <20160329164233.GB11035@twins.programming.kicks-ass.net>
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On 03/29/2016 12:42 PM, Peter Zijlstra wrote:
> On Tue, Mar 29, 2016 at 05:39:35PM +0200, Peter Zijlstra wrote:
>> On Tue, Mar 22, 2016 at 01:46:43PM -0400, Waiman Long wrote:
>>> Ding Tianhong reported a live-lock situation where a constant stream
>>> of incoming optimistic spinners blocked a task in the wait list from
>>> getting the mutex.
>>>
>>> This patch attempts to fix this live-lock condition by enabling the
>>> woken task in the wait queue to enter into an optimistic spinning
>>> loop itself in parallel with the regular spinners in the OSQ. This
>>> should prevent the live-lock condition from happening.
>> I would very much like a few words on how fairness is preserved.
>>
>> Because while the waiter remains on the wait_list while it spins, and
>> therefore unlock()s will only wake it, and we'll only contend with the
>> one waiter, the fact that we have two spinners is not fair or starvation
>> proof at all.
> Alternatively, we can say this is good enough until proven deficient,
> but then we should still very much document this.

Yes, we can certainly do that.

Cheers,
Longman