linux-kernel - Re: [PATCH 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes

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Message-ID: <c4d005b7-4b5a-3d21-79ba-99e24fb68f4f@vmware.com>
Date:   Wed, 13 Jun 2018 10:34:02 +0200
From:   Thomas Hellstrom <thellstrom@...are.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        Davidlohr Bueso <dave@...olabs.net>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Josh Triplett <josh@...htriplett.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        linux-doc@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH 1/2] locking: Implement an algorithm choice for Wound-Wait
 mutexes

On 06/13/2018 09:54 AM, Greg Kroah-Hartman wrote:
> On Wed, Jun 13, 2018 at 09:47:44AM +0200, Thomas Hellstrom wrote:
>>   -----
>>   
>> +The algorithm (Wait-Die vs Wound-Wait) is chosen using the _is_wait_die
>> +argument to DEFINE_WW_CLASS(). As a rough rule of thumb, use Wound-Wait iff you
>> +typically expect the number of simultaneous competing transactions to be small,
>> +and the rollback cost can be substantial.
>> +
>>   Three different ways to acquire locks within the same w/w class. Common
>>   definitions for methods #1 and #2:
>>   
>> -static DEFINE_WW_CLASS(ww_class);
>> +static DEFINE_WW_CLASS(ww_class, false);
> Minor nit on the api here.  Having a "flag" is a royal pain.  You have
> to go and look up exactly what that "true/false" means every time you
> run across it in code to figure out what it means.  Don't do that if at
> all possible.
>
> Make a new api:
> 	DEFINE_WW_CLASS_DIE(ww_class);
> instead that then wraps that boolean internally to switch between the
> different types.  That way the api is "self-documenting" and we all know
> what is going on without having to dig through a header file.
>
> thanks,
>
> greg k-h

Good point. I'll update in a v2.

Thanks,

Thomas