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Date: Fri, 22 Jan 2016 08:41:41 -0500
From: Waiman Long <waiman.long@hpe.com>
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To: Peter Zijlstra <peterz@infradead.org>
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>,
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        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.
References: <56A0A4ED.3070308@huawei.com> <56A1638A.7050202@hpe.com> <20160122085422.GO6357@twins.programming.kicks-ass.net>
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On 01/22/2016 03:54 AM, Peter Zijlstra wrote:
> 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.

Yes, that is optional. I put it there just to make it is more likely for 
the waiter spinner to get the lock. Without that, the chance will be 
50/50 on average. I can certainly take that out.

> 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.

As I said in the previous mail, I do intend to refactor it before 
sending out the official patch.

Cheers,
Longman