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

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Date:   Wed, 13 Jun 2018 09:54:11 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Thomas Hellstrom <thellstrom@...are.com>
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 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