linux-kernel - Re: [RFC][PATCH 4/4] Rename lock_cpu_hotplug/unlock_cpu

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Date:	Fri, 25 Aug 2006 00:55:26 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	ego@...ibm.com, rusty@...tcorp.com.au, torvalds@...l.org,
	akpm@...l.org, linux-kernel@...r.kernel.org, arjan@...el.linux.com,
	mingo@...e.hu, davej@...hat.com, dipankar@...ibm.com,
	vatsa@...ibm.com, ashok.raj@...el.com
Subject: Re: [RFC][PATCH 4/4] Rename lock_cpu_hotplug/unlock_cpu_hotplug

Arjan van de Ven wrote:
> On Thu, 2006-08-24 at 19:33 +0530, Gautham R Shenoy wrote:
> 
>>On Thu, Aug 24, 2006 at 01:00:00PM +0200, Arjan van de Ven wrote:
>>
>>>On Thu, 2006-08-24 at 16:04 +0530, Gautham R Shenoy wrote:
>>>
>>>>
>>>>This patch renames lock_cpu_hotplug to cpu_hotplug_disable and
>>>>unlock_cpu_hotplug to cpu_hotplug_enable throughout the kernel.
>>>
>>>Hi,
>>>
>>>to be honest I dislike the new names too. You turned it into a refcount,
>>>which is good, but the normal linux name for such refcount functions is
>>>_get and _put.....  and in addition the refcount technically isn't
>>>hotplug specific, all you want is to keep the kernel data for the
>>>processor as being "used", so cpu_get() and cpu_put() would sound
>>>reasonable names to me, or cpu_data_get() cpu_data_put().
>>
>>Thus, choice of 'cpu_hotplug_disable' and 'cpu_hotplug_enable'
>>was determined on the basis of its purpose, as in *what* it does 
>>as opposed to *how* it does it. :)
> 
> 
> well.. it comes down to the difference of locking to protect data versus
> locking to protect against a specific piece of code. Almost always the
> later turns out to be a mistake...

But it is not protecting a cpu from going away, it is protecting ALL
cpus from coming or leaving. In that respect it is much more like a
cpu_online_map lock rather than a data structure refcount.

It really is just like a reentrant rw semaphore... I don't see the
point of the name change, but I guess we don't like reentrant locks so
calling it something else might go down better with Linus ;)

-- 
SUSE Labs, Novell Inc.
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