linux-kernel - Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list is not NULL.

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Message-ID: <20160122085422.GO6357@twins.programming.kicks-ass.net>
Date:	Fri, 22 Jan 2016 09:54:22 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <waiman.long@....com>
Cc:	Ding Tianhong <dingtianhong@...wei.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <dave@...olabs.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <Will.Deacon@....com>,
	Jason Low <jason.low2@...com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Waiman Long <Waiman.Long@...com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
 is not NULL.

On Thu, Jan 21, 2016 at 06:02:34PM -0500, Waiman Long wrote:
> This patch attempts to fix this live-lock condition by enabling the
> a woken task in the wait list to enter optimistic spinning loop itself
> with precedence over the ones in the OSQ. This should prevent the
> live-lock
> condition from happening.


So I think having the top waiter going back in to contend on the OSQ is
an excellent idea, but I'm not sure the wlh_spinning thing is important.

The OSQ itself is FIFO fair, and the waiters retain the wait_list
position. So having the top wait_list entry contending on the OSQ
ensures we cannot starve (I think).

Also, as Davidlohr said, we cannot copy/paste this much code.