Message-ID: <20090121115744.GB22054@elte.hu>
Date:	Wed, 21 Jan 2009 12:57:44 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Vegard Nossum <vegard.nossum@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: lockdep and debug objects together are broken?


* Nick Piggin <npiggin@...e.de> wrote:

> On Wed, Jan 21, 2009 at 12:42:29PM +0100, Ingo Molnar wrote:
> > 
> > * Nick Piggin <npiggin@...e.de> wrote:
> > 
> > > On Tue, Jan 20, 2009 at 10:11:47PM +0100, Vegard Nossum wrote:
> > > > On Tue, Jan 20, 2009 at 9:55 AM, Nick Piggin <npiggin@...e.de> wrote:
> > > > > Hi,
> > > > >
> > > > > I've had a problem frustrating my testing because lockdep was silently turning
> > > > > itself off... I patched out the code to disable lockdep after the first error,
> > > > > and it started showing up weird errors. kernel/fork.c:990 seemed to be the
> > > > > first to trigger (hard irqs disabled) from a call_usermodehelper call. Later,
> > > > > migration thread was reported to try to unlock rq->lock although it was
> > > > > holding no locks. Then init was reported to return to userspace without
> > > > > releasing an objectdebug hash lock.
> > > > >
> > > > > All that went away and everything seemed to work properly with debug objects
> > > > > configured out.
> > > > >
> > > > > I didn't get too far in trying to debug the problem. But it should be easy
> > > > > enough to reproduce (if not, I can post traces or test patches).
> > > > 
> > > > I just built a kernel with lockdep and debugobjects enabled, and
> > > > everything seemed fine. I think you should post your kernel version,
> > > > config, and the lockdep patch (if needed -- it didn't seem to turn
> > > > itself off here).
> > > 
> > > Are you sure? Ie. sysrq+D a still works properly? In that case, you
> > > wouldn't need the lockdep patch because it just prevents the feature from being
> > > switched off.
> > > 
> > > I'll have to dig a bit further, then. The annoying thing is that
> > > lockdep turns itself off at the drop of a hat (and this particular
> > > problem seems to happen without any backtraces), so it invalidates
> > > all your lockdep testing if you don't realise it has turned itself
> > > off.
> > > 
> > > Is there a way to re-arm lockdep? That would be neat.
> > 
> > Not at the moment, and it looks somewhat complicated. All lock state 
> > freezes the moment lockdep disarms itself. That's very much a key design 
> > element: rarely will you see any real lockdep-inflicted crash - even if it 
> > has a bug it is self-disabling itself and running for the door very 
> > efficiently.
> 
> Lockdep isn't exactly for production systems though, is it? If you want 
> to debug some problem but you have other code (that you don't have 
> knowledge to debug) is switching it off...
> 
> Also, I'd guess that most bugs in lockdep would probably fall pretty 
> neatly into either the "pretty harmless" or "completely take down the 
> system" categories ;)

i think lockdep could be expanded into production use via code patching 
techniques.

So in that sense the rearm bit could be useful - it would give us a 
lockdep variant that would run for the first 5 minutes of uptime (where 
90% of all lockdep reports trigger: lockdep maps the dependencies very 
quickly) - and could turn itself off after that, and patch out / disable 
its callbacks.

The memory footprint would still remain, but that is not nearly as much of 
a problem for production systems as runtime overhead.

	Ingo
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