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

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Date:	Thu, 04 Apr 2013 18:49:18 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Daniel Vetter <daniel@...ll.ch>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	linux-arch@...r.kernel.org, daniel.vetter@...ll.ch, x86@...nel.org,
	linux-kernel@...r.kernel.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 Thu, 2013-04-04 at 15:31 +0200, Daniel Vetter wrote:
> We've discussed this approach of using (rt-prio, age) instead of just
> age
> to determine the the "oldness" of a task for deadlock-breaking with
> -EAGAIN. The problem is that through PI boosting or normal rt-prio
> changes
> while tasks are trying to acquire ww_mutexes you can create acyclic
> loops
> in the blocked-on graph of ww_mutexes after the fact and so cause
> deadlocks. So we've convinced ourselves that this approche doesn't
> work.

Could you pretty please draw me a picture, I'm having trouble seeing
what you mean.

AFAICT as long as you boost Y while its the lock owner things should
work out, no?



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