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

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Date:	Thu, 4 Apr 2013 18:59:10 +0200
From:	Daniel Vetter <daniel@...ll.ch>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	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>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [PATCH v2 2/3] mutex: add support for reservation style locks, v2

On Thu, Apr 4, 2013 at 6:38 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, 2013-04-04 at 15:31 +0200, Daniel Vetter wrote:
>> Hm, I guess your aim with the TASK_DEADLOCK wakeup is to bound the
>> wait
>> times of older task.
>
> No, imagine the following:
>
> struct ww_mutex A, B;
> struct mutex C;
>
>         task-O  task-Y  task-X
>                 A
>                 B
>                         C
>                 C
>         B
>
> At this point O finds that Y owns B and thus we want to make Y 'yield'
> B to make allow B progress. Since Y is blocked, we'll send a wakeup.
> However Y is blocked on a different locking primitive; one that doesn't
> collaborate in the -EDEADLK scheme therefore we don't want the wakeup to
> succeed.

Yeah, I've thought about this some more and the special sleep state
seems to be only required to ensure we don't cause spurious wakeups
for other any other reasons a task blocks. But I think we can use that
kick-current-holder approach to ensure that older tasks get the lock
in a more timely fashion than the current code. I've tried to detail
it a bit with another 3 task example - that seems to be the point
where the fun starts ;-)
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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