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

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Date:	Tue, 29 Mar 2016 18:42:33 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
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 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.