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

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Message-ID: <20140513203619.GV18164@linux.vnet.ibm.com>
Date:	Tue, 13 May 2014 13:36:19 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	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, May 13, 2014 at 04:20:41PM -0400, Steven Rostedt wrote:
> 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?

I second this approach!  The kernel environment makes it -much- easier
to force races and other conditions, which turns into much simpler and
more effective tests.

							Thanx, Paul

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