linux-kernel - Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest

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Date:	Mon, 4 Aug 2008 19:27:28 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Dave Jones <davej@...hat.com>, Roland Dreier <rdreier@...co.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, jeremy@...p.org,
	hugh@...itas.com, mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 7/7] lockdep: spin_lock_nest_lock()

On Mon, Aug 04, 2008 at 06:38:55PM +0200, Peter Zijlstra wrote:
> You also need CONFIG_PROVE_LOCKING, but I'll assume that's set too.

Sorry, that wasn't the problem, but my current testing passed because
of another error... to test this report I rebuilt a kvm configured for
rhel not for mainline so it wasn't the right test...

When "long ago" I tested that this was working fine (actually when
Andrew asked me), I guess lockdep was working because the
implementation with the vmalloc array was slightly different,
otherwise I don't know. I'm fairly certain that it worked fine at some
point, and I didn't expect the refactoring to generate false positives.

> Dave Jones just handed me:
> 
>   https://bugzilla.redhat.com/show_bug.cgi?id=457779

I can reproduce this now yes after a 'make sync'.

Obviously this is a bug in lockdep that it trips over this otherwise
if lockdep was right the kernel should deadlock while this is just a
false positive and everything runs fine.

I assume it can't understand the spinlock address is different (I
think it uses the address as key only for static locks), so I wonder
if you could call print_deadlock_bug() from the failure path of the
spinlock to solve this?

Do things like double_rq_lock works just because rq1 and rq2 don't
have the same name like in my case where all locks are called "mapping"->?

> David Miller just did a patch that might fix that.

Woow cool, after 11 months I lost any hope that lockdep could ever
work in that environment... Was it an actual bug or is this some way
to lower the complexity of the graph build?
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