linux-kernel - Re: [Linaro-mm-sig] [PATCH v3 2/3] mutex: add support for wound/wait style locks, v3

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Message-ID: <CAKMK7uF=0aW4dY_0bmUox1N-kmQvXRr2-x3NBjL2VGO4grW6fQ@mail.gmail.com>
Date:	Thu, 23 May 2013 12:45:00 +0200
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, linux-arch@...r.kernel.org,
	x86@...nel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	dri-devel <dri-devel@...ts.freedesktop.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	rob clark <robclark@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Dave Airlie <airlied@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [Linaro-mm-sig] [PATCH v3 2/3] mutex: add support for wound/wait
 style locks, v3

On Thu, May 23, 2013 at 11:13 AM, Maarten Lankhorst
<maarten.lankhorst@...onical.com> wrote:
>> 2. Do you really want to drop the *_slow variants?
>> Doing so might reduce debugging slightly. I like method #2 in ww-mutex-design.txt, it makes it very clear why you
>> would handle the *_slow case differently anyway.
> As you pointed out, we wouldn't lose much debugging information.
> The same checks could be done in the normal variant with
> WARN_ON(ctx->lock && ctx->lock != lock);
> WARN_ON(ctx->lock && ctx->acquired > 0);

s/lock/contending_lock/ I guess. But yeah, I should have more
carefully read Peter's suggestion to fold in some of the ww_slow debug
checks, we can indeed keep the important debug checks even when
dropping slow. Silly me should be less sloppy.

> But it boils down to ww_mutex_lock_slow returning void instead of int __must_check from ww_mutex_lock.
>
> Maybe add inlines for *_slow, that use the ww_mutex_lock functions, and check ctx->lock == lock in debugging mode?

So either we keep the _slow versions or drop the __must_check for
ww_mutex_lock. In both cases the ww mutex user needs to think a bit
what to do, and I don't there's much we can do in the implementation
(beside all the existing debug support we have) to help. So now I'm
leaning more towards dropping the _slow variants to avoid interface
proliferation.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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