linux-kernel - Re: [patch 1/3] rtmutex: Add missing deadlock check

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Message-ID: <20140513162041.3405b668@gandalf.local.home>
Date:	Tue, 13 May 2014 16:20:41 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Dave Jones <davej@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Davidlohr Bueso <davidlohr@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Clark Williams <williams@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Roland McGrath <roland@...k.frob.com>,
	Carlos ODonell <carlos@...hat.com>,
	Jakub Jelinek <jakub@...hat.com>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [patch 1/3] rtmutex: Add missing deadlock check

On Tue, 13 May 2014 21:42:54 +0200 (CEST)
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Tue, 13 May 2014, Peter Zijlstra wrote:
> > 
> > Now, if you and Steve get this sorted, nothing really happened except
> > that Thomas got grumpy, which is entirely normal, what else would he be?
> > :-)
> 
> Who is that grumpy Thomas dude, should I know him?
> 
> Lai, Steven,
> 
> before you waste lots of time on the tester, I want to look at it
> whether we can simplify it or even rewrite it from scratch. I glanced
> at it today and I really can't remember what kind of substances were
> involved when I wrote this almost a decade ago.

Thank God. /me removes the ton of trace_printk()s in the code as well
as all the trace_marker.write("%s" %(line)) from the test to figure out
what was going on.

> 
> The whole schedule_rt_mutex mechanism was mostly done to create
> controlled lock stealing scenarios and deal with the BKL
> oddities.
> 
> With Lai's simplification and the demise of BKL I'm quite sure we do
> not need it anymore.
> 
> So we can just get rid of the complexity in schedule_rt_mutex() and
> replace it with a simple:
> 
>     while (!td->continue)
>     	  schedule();
> 
> That would also make the teardown and reset of the whole thing
> manageable. Right now it's easy to create a situation where unrolling
> stuff gets almost impossible except by pushing the reset button.
> 
> The state readouts can be done directly via the rtmutexes and the task
> structs.
> 
> Thoughts?
> 

What about having a module that creates a bunch of threads and forces
all the scenarios that we want to test? Wouldn't it be easier to do
than to have a userspace interface to dictate commands to the kernel?

-- Steve
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