linux-kernel - Re: remove cpu hotplug bustification in cpufreq.

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Message-Id: <1153643380.7359.24.camel@laptopd505.fenrus.org>
Date:	Sun, 23 Jul 2006 10:29:40 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	Ingo Molnar <mingo@...e.hu>, ashok.raj@...el.com,
	linux-kernel@...r.kernel.org, davej@...hat.com,
	Linus Torvalds <torvalds@...l.org>
Subject: Re: remove cpu hotplug bustification in cpufreq.

Hi,

> > Well, I just got Ashok's trial patches which turns the thing into a rwsem 
> > as I outlined earlier.
> 
> Mark my words ;)
> 
> > I'll try them out. If they don't work, we should just delete the lock and 
> > go totally back to square 1.
> 
> rwsem conversion has the potential to merely hide the problem.  Ingo, does
> lockdep detect recursive down_read()?

lockdep detects and warns about those.

I think we're about equally skeptical about this; I'm extremely hesitant
about any place in the kernel that uses rwsems for anything other than a
performance tweak. I've ended up with a mental model of rwsems that
basically comes down to "you need to be able to replace it with a mutex
without breaking correctness". Now of course that model is somewhat of
an oversimplification, but not by that much...

Greetings,
   Arjan van de Ven
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