linux-kernel - Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

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Message-ID: <378093ad-46cc-7ecb-5a06-1e22ee5ce4a1@redhat.com>
Date:   Wed, 17 Jul 2019 09:35:46 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Alex Kogan <alex.kogan@...cle.com>, linux@...linux.org.uk,
        mingo@...hat.com, will.deacon@....com, arnd@...db.de,
        linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, bp@...en8.de,
        hpa@...or.com, x86@...nel.org, guohanjun@...wei.com,
        jglauber@...vell.com, steven.sistare@...cle.com,
        daniel.m.jordan@...cle.com, dave.dice@...cle.com,
        rahul.x.yadav@...cle.com
Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock

On 7/17/19 3:44 AM, Peter Zijlstra wrote:
> On Tue, Jul 16, 2019 at 10:16:29PM -0400, Waiman Long wrote:
>>  A simple graphic to illustrate those queues will help too, for example
> Very much yes!
>
>> /*
>>  * MCS lock holder
>>  * ===============
>>  *    mcs_node
>>  *   +--------+      +----+         +----+
>>  *   | next   | ---> |next| -> ...  |next| -> NULL  [Main queue]
>>  *   | locked | -+   +----+         +----+
>>  *   +--------+  |
>>  *               |   +----+         +----+
>>  *               +-> |next| -> ...  |next| -> X     [Secondary queue]
>>  *    cna_node       +----+         +----+
>>  *   +--------*                       ^
>>  *   | tail   | ----------------------+
>>  *   +--------*   
> Almost; IIUC that cna_node is the same as the one from locked, so you
> end up with something like:
>
>>  *    mcs_node
>>  *   +--------+      +----+         +----+
>>  *   | next   | ---> |next| -> ...  |next| -> NULL  [Main queue]
>>  *   | locked | -+   +----+         +----+
>>  *   +--------+  |
>>  *               |   +---------+         +----+
>>  *               +-> |mcs::next| -> ...  |next| -> NULL     [Secondary queue]
>>  *                   |cna::tail| -+      +----+
>>  *                   +---------+  |        ^
>>  *                                +--------+
>>  *
>>  * N.B. locked = 1 if secondary queue is absent.
>>  */

Yes, you are right. Thanks for the correction.

Cheers,
Longman