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Date: Wed, 14 Mar 2007 10:50:05 +1100
From: Paul Mackerras <paulus@samba.org>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>, Andi Kleen <andi@firstfloor.org>,
       "Marcin 'Qrczak' Kowalczyk" <qrczak@knm.org.pl>,
       linux-kernel@vger.kernel.org
Subject: Re: _proxy_pda still makes linking modules fail
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Jeremy Fitzhardinge writes:

> Or do you mean that if you have:
> 
> 	preempt_disable();
> 	use_my_percpu++;
> 	preempt_enable();
> 	// switch cpus
> 	preempt_disable();
> 	use_my_percpu++;
> 	preempt_enable();
> 
> then it will still use the old pointer to use_my_percpu?

Yes.  It can, and sometimes does.  There's no way (that I know of) to
tell gcc "all my __thread variables might have moved to a different
address".

> In principle gcc could CSE the value of smp_processor_id() across a cpu
> change in the same way.

There it's easier to make gcc do what we want, because we can use a
barrier or a volatile.  The difference is that smp_processor_id() is
ultimately the value of something, not the address of something.  We
can tell gcc "values might have changed" but have no way to say
"addresses might have changed".

Paul.