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

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Message-ID: <d120d5000609150656v544ca0d6vcf93e349508dedd8@mail.gmail.com>
Date:	Fri, 15 Sep 2006 09:56:46 -0400
From:	"Dmitry Torokhov" <dmitry.torokhov@...il.com>
To:	"Jiri Kosina" <jikos@...os.cz>
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 9/15/06, Dmitry Torokhov <dmitry.torokhov@...il.com> wrote:
> On 9/15/06, Jiri Kosina <jikos@...os.cz> wrote:
> > 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.
> >
>
> But nested semantics breaks the notion of the locks belonging to the
> same pool so both solutions have tradeoffs. I could use either one of
> these as long as details are hidden and callers do not have to care.
>

One more thing I forgot to add - how will we deal with lockdep in
cases when we have X-over-Y-over-X protocol, when there is no tight
coupling between the X parts so it is impossible to know when to apply
special marking on the lock or callers?

-- 
Dmitry
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