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Date: Fri, 22 Jan 2016 09:54:22 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Waiman Long <waiman.long@hpe.com>
Cc: Ding Tianhong <dingtianhong@huawei.com>, Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        "Paul E. McKenney" <paulmck@us.ibm.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <Will.Deacon@arm.com>, Jason Low <jason.low2@hp.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH RFC] locking/mutexes: don't spin on owner when wait list
 is not NULL.
Message-ID: <20160122085422.GO6357@twins.programming.kicks-ass.net>
References: <56A0A4ED.3070308@huawei.com>
 <56A1638A.7050202@hpe.com>
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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.