Re: _proxy_pda still makes linking modules fail

[Jeremy Fitzhardinge <jeremy@goop.org>]

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