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Date:   Wed, 17 Jul 2019 09:44:35 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Alex Kogan <alex.kogan@oracle.com>, linux@armlinux.org.uk,
        mingo@redhat.com, will.deacon@arm.com, arnd@arndb.de,
        linux-arch@vger.kernel.org, linux-arm-kernel@lists.infradead.org,
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        hpa@zytor.com, x86@kernel.org, guohanjun@huawei.com,
        jglauber@marvell.com, steven.sistare@oracle.com,
        daniel.m.jordan@oracle.com, dave.dice@oracle.com,
        rahul.x.yadav@oracle.com
Subject: Re: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow
 path of qspinlock
Message-ID: <20190717074435.GU3419@hirez.programming.kicks-ass.net>
References: <20190715192536.104548-1-alex.kogan@oracle.com>
 <20190715192536.104548-4-alex.kogan@oracle.com>
 <9fa54e98-0b9b-0931-db32-c6bd6ccfe75b@redhat.com>
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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.
> ?*/