linux-kernel - Re: [patch] Reorganize the cpufreq cpu hotplug locking to not be totally bizare

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Date:	Wed, 26 Jul 2006 23:03:06 +0200
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	vatsa@...ibm.com
Cc:	Linus Torvalds <torvalds@...l.org>, Dave Jones <davej@...hat.com>,
	Ingo Molnar <mingo@...e.hu>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	Ashok Raj <ashok.raj@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>
Subject: Re: [patch] Reorganize the cpufreq cpu hotplug locking to not be
	totally bizare

On Wed, 2006-07-26 at 13:42 -0700, Srivatsa Vaddagiri wrote:
> On Wed, Jul 26, 2006 at 09:42:34PM +0200, Arjan van de Ven wrote:
> > As a quick hack I made non-lock_cpu_hotplug()'ing versions of the 3 key
> > workqueue functions (patch below). It works, it's correct, it's just so
> > ugly that I'm almost too ashamed to post it. I haven't found a better
> > solution yet though... time to take a step back I suppose.
> 
> My worry is that such special cases might be needed in more places as we
> discover further or as code evolves. Fundamentally looks like the locked and 
> unlocked paths of the kernel cannot be separated so well because of interaction 
> between subsystems. /me thinks rwsem seems to be a sane thing to go after.

rwsems unfortunately help you zilch; an rwsem is just a mutex with a
performance tweak, but from the deadlock perspective it's really a
mutex.

I'm really starting to feel that the hotplug lock would have been better
of being a refcount (with a waitqueue for zero) than a lock. While
"refcount+waitqueue" sort of IS a lock, the semantics make more sense
imo...

Greetings,
   Arjan van de Ven
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