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SubjectRe: [PATCH v3 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock
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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

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