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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
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