linux-kernel - Re: [RFC][PATCH 0/4] Redesign cpu

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Date:	Sun, 27 Aug 2006 18:57:44 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...l.org>
CC:	dipankar@...ibm.com, Linus Torvalds <torvalds@...l.org>,
	Dave Jones <davej@...hat.com>, ego@...ibm.com,
	rusty@...tcorp.com.au, linux-kernel@...r.kernel.org,
	arjan@...el.linux.com, mingo@...e.hu, vatsa@...ibm.com,
	ashok.raj@...el.com
Subject: Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

Andrew Morton wrote:
> On Sun, 27 Aug 2006 12:41:16 +0530
> Dipankar Sarma <dipankar@...ibm.com> wrote:

>>Is this too difficult for people to follow ?
> 
> 
> Apparently.  What's happening is that lock_cpu_hotplug() is seen as some
> amazing thing which will prevent an *event* from occurring.

It prevents the event from occurring as much as a lock taken in the
prepare notifier does, right? Or am I misunderstanding something?

> 
> There's an old saying "lock data, not code".  What data is being locked
> here?  It's the subsystem's per-cpu resources which we want to lock.  We
> shouldn't consider the lock as being some way of preventing an event from
> happening.

I agree. Where possible these things should be very simple either with
the per-cpu macros, or using local locking (versus one's notifier).

I think there can be some valid use of the hotplug lock when working
with cpumasks rather than individual CPUs (or if you simply want to
prevent a cpu from going down while programming hardware) and having a
new lock and new notifier is too heavyweight. Or do we want to move
away from the hotplug lock completely?

Hmm, so I don't know if I like the idea of a reentrant rwmutex for the
hotplug lock so it can be sprinkled everywhere... call it a refcount
or not it smells slightly BKLish (looks easy but it could be a
nightmare to audit).

-- 
SUSE Labs, Novell Inc.
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