Messages in this thread | | | Date | Tue, 13 Mar 2007 08:31:53 -0700 | From | Jeremy Fitzhardinge <> | 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |