linux-kernel - RE: [PATCH 13/16] percpu: remove per_cpu_

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Message-ID: <alpine.DEB.1.10.0910141445110.30078@gentwo.org>
Date:	Wed, 14 Oct 2009 14:51:02 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	"Luck, Tony" <tony.luck@...el.com>
cc:	Tejun Heo <tj@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"cebbert@...hat.com" <cebbert@...hat.com>
Subject: RE: [PATCH 13/16] percpu: remove per_cpu__ prefix.

On Wed, 14 Oct 2009, Luck, Tony wrote:

> > we would still have to use per cpu operations to get to the contents of
> > these variables.
>
> That's good.
>
> > Hope that addresses your concerns.
>
> But then I don't understand the original patch that was going to do:
>
> > -#define __ia64_per_cpu_var(var)	per_cpu__##var
> > +#define __ia64_per_cpu_var(var)	var
>
> Presumably all actual use of __ia64_per_cpu_var is being replaced
> by some other "per cpu operations"?

Hmmm... Right. IA64 is a special case because the access of the per cpu
variable at a specific address causes per cpu TLBs to do the relocation.

Other platforms have to add a per cpu specific offset to a variable to get
the right per cpu variable.

As a result IA64 strictly does not need this_cpu_read() and
this_cpu_write(). However, not using the operations is going to cause
the sparse annotation by Tejun to trigger errors. this_cpu_read() is
likely a noop for IA64 that just changes the annotations so that sparse
warnings do not trigger. Tejun?

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