linux-kernel - Re: [PATCH v3 2/3] locking/mutex: Enable optimistic spinning of woken task in wait queue

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Message-ID: <56FD8B3E.3000306@hpe.com>
Date:	Thu, 31 Mar 2016 16:40:30 -0400
From:	Waiman Long <waiman.long@....com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	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>,
	Davidlohr Bueso <dave@...olabs.net>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH v3 2/3] locking/mutex: Enable optimistic spinning of woken
 task in wait queue

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