linux-kernel - Re: [PATCH v2 2/3] mutex: add support for reservation style locks, v2

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Message-ID: <CAKMK7uHPtR1m_GYTH2xmnfo-rCJUCvz3_ci40jp9zOWTNpzsvw@mail.gmail.com>
Date:	Tue, 2 Apr 2013 19:30:57 +0200
From:	Daniel Vetter <daniel.vetter@...ll.ch>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	x86@...nel.org, dri-devel@...ts.freedesktop.org,
	linaro-mm-sig@...ts.linaro.org, robclark@...il.com,
	tglx@...utronix.de, mingo@...e.hu, linux-media@...r.kernel.org
Subject: Re: [PATCH v2 2/3] mutex: add support for reservation style locks, v2

On Tue, Apr 2, 2013 at 6:59 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>> > Also, is there anything in CS literature that comes close to this? I'd
>> > think the DBMS people would have something similar with their
>> > transactional systems. What do they call it?
>
>> I didn't study cs, but judging from your phrasing I guess you mean you
>> want me to call it transaction_mutexes instead?
>
> Nah, me neither, I just hate reinventing names for something that's
> already got a perfectly fine name under which a bunch of people know
> it.
>
> See the email from Daniel, apparently its known as wound-wait deadlock
> avoidance -- its actually described in the "deadlock" wikipedia
> article.
>
> So how about we call the thing something like:
>
>   struct ww_mutex; /* wound/wait */
>
>   int mutex_wound_lock(struct ww_mutex *); /* returns -EDEADLK */
>   int mutex_wait_lock(struct ww_mutex *);  /* does not fail */

I'm not sold on this prefix, since wound-wait is just the
implementation detail of how it detects/handles deadlocks. For users a
really dumb strategy of just doing a mutex trylock and always
returning -EAGAIN if that fails (together with a msleep(rand) in the
slowpath) would have the same interface. Almost at least, we could
ditch the ticket - but the ticket is used as a virtual lock for the
lockdep annotation, so ditching it would also reduce lockdep
usefulness (due to all those trylocks). So in case we ever switch the
deadlock/backoff algo ww_ would be a bit misleading.

Otoh reservation_ is just what it's used for in graphics-land, so not
that much better. I don't really have a good idea for what this is
besides mutexes_with_magic_deadlock_detection_and_backoff. Which is a
bit long.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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