Re: [PATCH 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes - Greg Kroah-Hartman

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Thomas Hellstrom <thellstrom@vmware.com>
Cc: dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Jonathan Corbet <corbet@lwn.net>,
	Gustavo Padovan <gustavo@padovan.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Sean Paul <seanpaul@chromium.org>,
	David Airlie <airlied@linux.ie>,
	Davidlohr Bueso <dave@stgolabs.net>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Kate Stewart <kstewart@linuxfoundation.org>,
	Philippe Ombredanne <pombredanne@nexb.com>,
	linux-doc@vger.kernel.org, linux-media@vger.kernel.org,
	linaro-mm-sig@lists.linaro.org
Subject: Re: [PATCH 1/2] locking: Implement an algorithm choice for Wound-Wait mutexes
Date: Wed, 13 Jun 2018 09:54:11 +0200	[thread overview]
Message-ID: <20180613075411.GA17681@kroah.com> (raw)
In-Reply-To: <20180613074745.14750-2-thellstrom@vmware.com>

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