linux-kernel - Re: [PATCH 0/3] Synaptics

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Message-ID: <Pine.LNX.4.64.0609151535190.2721@twin.jikos.cz>
Date:	Fri, 15 Sep 2006 15:38:51 +0200 (CEST)
From:	Jiri Kosina <jikos@...os.cz>
To:	Dmitry Torokhov <dmitry.torokhov@...il.com>
cc:	Arjan van de Ven <arjan@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 0/3] Synaptics - fix lockdep warnings

On Fri, 15 Sep 2006, Dmitry Torokhov wrote:

> I understand what Ingo is saying about detecting deadlocks across the 
> pool of locks of the same class not waiting till they really clash, it 
> is really useful. I also want to make my code as independent of lockdep 
> as possible. Having a speciall marking on the locks themselves (done 
> upon creation) instead of altering call sites is the cleanest way IMHO. 
> Can we have a flag in the lock structure that would tell lockdep that it 
> is OK for the given lock to be taken several times (i.e. the locks are 
> really on the different objects)? This would still allow to detect 
> incorrect locking across different classes.

Yes, but unfortunately marking the lock as 'can-be-taken-multiple-times' 
is weaker than using the nested locking provided by lockdep.

i.e. if you mark a lock this way, it opens door for having deadlock, which 
won't be detected by lockdep. This will happen if the code, by mistake, 
really takes the _very same_ lock twice. lockdep will not be able to 
detect this, when the lock is marked in a way you propose, but is able to 
detect this when using the nested semantics.

-- 
JiKos.
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