linux-kernel - Re: _proxy_pda still makes linking modules fail

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Date:	Wed, 14 Mar 2007 12:12:49 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Paul Mackerras <paulus@...ba.org>,
	Andi Kleen <andi@...stfloor.org>,
	Marcin 'Qrczak' Kowalczyk <qrczak@....org.pl>,
	linux-kernel@...r.kernel.org
Subject: Re: _proxy_pda still makes linking modules fail

On Tue, 2007-03-13 at 08:31 -0700, Jeremy Fitzhardinge wrote:
> Paul Mackerras wrote:
> > There is a fundamental problem with using __thread, which is that gcc
> > assumes that the addresses of __thread variables are constant within
> > one thread, and that therefore it can cache the result of address
> > calculations.  However, with preempt, threads in the kernel can't rely
> > on staying on one cpu, and therefore the addresses of per-cpu
> > variables can change.  There appears to be no way to tell gcc to drop
> > all cached __thread variable address calculations at a given point
> > (e.g. when enabling or disabling preemption).  That is basically why I
> > gave up on using __thread for per-cpu variables on powerpc.

[ Thanks for the enlightenment, Paul ]

> Doesn't that fall under the general class of "you have to be pinned to a
> particular cpu in order to meaningfully use per-cpu variables"?

No, it makes assumptions about the *address* of a per-cpu variable not
changing, even across barriers.

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

No, this is why preempt_enable and the like are memory barriers.
Rusty.


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