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

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Date:	Tue, 13 Mar 2007 08:31:53 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Paul Mackerras <paulus@...ba.org>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	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

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.
>   

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"?

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?

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

    J
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