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

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Date:   Wed, 17 Jul 2019 10:42:26 -0400
From:   Alex Kogan <alex.kogan@...cle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.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


>>  *    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.
>>  */


I would change mcs_node to cna_node, next to mcs::next and locked to mcs::locked
in the very first node. Other than that, this is great. Thanks, Peter and Longman!

I should probably stick this graphic in the comment at the top of the file, 
instead of a comment to find_successor() (or whatever this function ends up 
being called).

— Alex