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Date: Thu, 14 Mar 2002 19:51:22 +0100
From: Andi Kleen <ak@suse.de>
To: davidm@hpl.hp.com
Cc: Andi Kleen <ak@suse.de>, Rusty Russell <rusty@rustcorp.com.au>,
        linux-kernel@vger.kernel.org, torvalds@transmeta.com, rth@twiddle.net
Subject: Re: [PATCH] 2.5.1-pre5: per-cpu areas
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On Thu, Mar 14, 2002 at 10:04:05AM -0800, David Mosberger wrote:
> How about the following proposal:
> 
> 	- taking the address of this_cpu(var) is never allowed (can be
>           enforced with RELOC_HIDE())
> 
> 	- taking the address of per_cpu(var, n) is always legal and
> 	  will return a pointer which will access CPU n's version of
> 	  the variable, no matter what CPU dereferences the pointer
> 
> Andi, I think this would take care of the x86-64 problem as well, right?

Yes, it would. 

It would be a bit more overhead for taking the address than a 
this_cpu_address(), because one would need to fetch the CPU number first
and do the arithmetic and the array reference instead of fetching the 
address directly. But I agree that per_cpu() has much cleaner semantics than
this_cpu_address() for addresses, so it is worth it.

When one considers preemptive kernels where you can lose your CPU anytime
then it makes even more sense. But then this_cpu is only safe there when
you turn off preemption or hold some lock or run in interrupt context.
Outside such regions per_cpu() seems to be safer. 

Thanks, 
-Andi