linux-kernel - Re: [PATCH v4 2/3] locking/mutex: Enable optimistic spinning of woken task in wait queue

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Date:	Tue, 9 Aug 2016 13:49:01 -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>,
	Imre Deak <imre.deak@...el.com>
Subject: Re: [PATCH v4 2/3] locking/mutex: Enable optimistic spinning of woken
 task in wait queue

On 08/08/2016 01:29 PM, Peter Zijlstra wrote:
> On Mon, Jul 18, 2016 at 04:39:25PM -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.
> No, two spinners are not in fact starvation proof. It makes the reported
> life-lock scenario much less likely, but it does not guarantee anything.

Yes, I should have said reducing the chance of live-locking.

>> +		/*
>> +		 * Optimistically spinning on the mutex without the wait lock
> There should either be a '.' at the end of that line, or the next line
> should not start with a capital.
>
> Also, I don't see how the two sentences are related, should they be in
> the same paragraph?

Sorry for the missing '.', and I will split it into 2 paragraphs.

Cheers,
Longman