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

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Message-Id: <20060726143357.2f0787e7.akpm@osdl.org>
Date:	Wed, 26 Jul 2006 14:33:57 -0700
From:	Andrew Morton <akpm@...l.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	vatsa@...ibm.com, torvalds@...l.org, davej@...hat.com,
	mingo@...e.hu, 76306.1226@...puserve.com, ashok.raj@...el.com,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] Reorganize the cpufreq cpu hotplug locking to not be
 totally bizare

On Wed, 26 Jul 2006 23:03:06 +0200
Arjan van de Ven <arjan@...ux.intel.com> wrote:

> 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...

The mistake in the above paragraph is its use of the term "the hotplug
lock".

Think.  We don't want to lock CPUs.  We don't want to block plug/unplug
events.

What we _do_ want is for subsystems to be able to guarantee the stability
of their per-cpu data and the coherency of that data with cpu_online_map
and cpu_present_map.

We should delete lock_cpu_hotplug() and start again.  Perhaps we can do
that post-2.6.18 if we can cobble the current stuff into some semi-working
state.  But I doubt if it's very important really - we have heaps of code
in there which is already racy wrt hotplug and adding a little more isn't
likely to hurt.

I count 187 instances of for_each_online_cpu(), and most of them are racy. 
There's just no way we can fix all these with lock_cpu_hotplug().  It
simply doesn't have a future.
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