Messages in this thread | | | Subject | Re: _proxy_pda still makes linking modules fail | From | Rusty Russell <> | Date | Wed, 14 Mar 2007 12:12:49 +1100 |
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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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