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

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Date:	Thu, 18 Apr 2013 20:37:02 +0300
From:	Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:	Daniel Vetter <daniel.vetter@...ll.ch>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-arch@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.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 Wed, Apr 17, 2013 at 09:08:17PM +0200, Daniel Vetter wrote:
> On Wed, Apr 10, 2013 at 12:28 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> > On Thu, Apr 04, 2013 at 06:41:02PM +0200, Peter Zijlstra wrote:
> >> On Thu, 2013-04-04 at 15:31 +0200, Daniel Vetter wrote:
> >> > The thing is now that you're not expected to hold these locks for a
> >> > long
> >> > time - if you need to synchronously stall while holding a lock
> >> > performance
> >> > will go down the gutters anyway. And since most current
> >> > gpus/co-processors
> >> > still can't really preempt fairness isn't that high a priority,
> >> > either.
> >> > So we didn't think too much about that.
> >>
> >> Yeah but you're proposing a new synchronization primitive for the core
> >> kernel.. all such 'fun' details need to be considered, not only those
> >> few that bear on the one usecase.
> >
> > Which bares the question, what other use cases are there?
> 
> Just stumbled over one I think: If we have a big graph of connected
> things (happens really often for video pipelines). And we want
> multiple users to use them in parallel. But sometimes a configuration
> change could take way too long and so would unduly stall a 2nd thread
> with just a global mutex, then per-object ww_mutexes would be a fit:
> You'd start with grabbing all the locks for the objects you want to
> change anything with, then grab anything in the graph that you also
> need to check. Thanks to loop detection and self-recursion this would
> all nicely work out, even for cyclic graphs of objects.

Indeed, that would make the locking for atomic modeset/page flip very
easy to handle, while still allowing the use of suitable fine grained
locks. I like the idea.

-- 
Ville Syrjälä
Intel OTC
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