linux-kernel - Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid starvation

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Date:   Fri, 26 Aug 2016 19:40:34 -0400
From:   Waiman Long <waiman.long@....com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Jason Low <jason.low2@....com>,
        Ding Tianhong <dingtianhong@...wei.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <Will.Deacon@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Imre Deak <imre.deak@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Terry Rudd <terry.rudd@....com>,
        "Paul E. McKenney" <paulmck@...ibm.com>,
        Jason Low <jason.low2@...com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: Re: [RFC][PATCH -v2 4/4] locking/mutex: Add lock handoff to avoid
 starvation

On 08/26/2016 11:18 AM, Peter Zijlstra wrote:
> On Fri, Aug 26, 2016 at 10:30:31AM -0400, Waiman Long wrote:
>> On 08/25/2016 06:23 PM, Peter Zijlstra wrote:
>>> You're right.. I'll try again tomorrow.
>> Actually, it is also OK to handoff from NULL=>new, but nothing else is
>> appropriate.
> I pushed out new patches to git, could you have a peek?
>
>   - killed another of that mutex abuse
>   - fixed ATOMIC_FETCH_OP for SH4
>   - split: rm arch/*/include/asm/mutex*.h include/asm-generic/mutex*.h
>     into a separate patch to aid in review
>   - fixed lockdep
>   - added an uncontended unlock fast path, to mirror the unconteded lock
>     fast path
>   - reworked the handoff stuff (again)
>
> I didn't try and be too clever, since its far too hot to think.
>
> Still need to look at adding spinning to the handoff case.
> Also need to look at writing (much) better changelogs, they stink.
>

I have looked at the handoff code and I didn't see any problem.

BTW, I also sent out a patch to add optimistic spinning to the waiter. 
Please let me know your thought on that.

Thanks,
Longman