linux-kernel - Re: [PATCH] workaround minor lockdep bug triggered by mm_take_all

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Message-ID: <20080805122024.GK12464@duo.random>
Date:	Tue, 5 Aug 2008 14:20:24 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Roland Dreier <rdreier@...co.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Dave Jones <davej@...hat.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, arjan <arjan@...radead.org>
Subject: Re: [PATCH] workaround minor lockdep bug triggered by
	mm_take_all_locks

On Tue, Aug 05, 2008 at 05:02:07AM -0700, Roland Dreier wrote:
> check_deadlock operates on classes of locks, so it can warn about
> potential deadlocks, eg if we have
> 
> 	foo(obj1, obj2)
> 	{
> 		lock(obj1);
> 		lock(obj2);
> 		...
> 
> then foo(obj, obj); is a deadlock but lockdep can warn about foo(obj,
> different_obj) without triggering the deadlock in reality.  Of course
> this leads to false positives, and we sometimes have to change correct
> code to help lockdep, but usually such rewriting leads to simpler
> clearer better locking anyway.

It surely doesn't lead to simpler and clearer better locking in the
case we're discussing here, and I don't know of other cases where it
leads to better locking but feel free to make examples.
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