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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
