Messages in this thread | | | Date | Tue, 08 Jul 2008 14:28:48 -0700 | From | Mike Travis <> | Subject | Re: [PATCH 1/1] x86: Change _node_to_cpumask_ptr to return const ptr |
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Vegard Nossum wrote: > On Tue, Jul 8, 2008 at 10:51 PM, Mike Travis <travis@sgi.com> wrote: >> Vegard Nossum wrote: >>> On Tue, Jul 8, 2008 at 8:05 PM, Mike Travis <travis@sgi.com> wrote: >>>>>> Note: I did not change node_to_cpumask_ptr() in include/asm-generic/topology.h >>>>>> as node_to_cpumask_ptr_next() does change the cpumask value. >>>>> Hmmm. Does it really? >>>>> >>>>> #define node_to_cpumask_ptr_next(v, node) \ >>>>> _##v = node_to_cpumask(node) >>>>> >>>>> This doesn't seem to modify it? >>>> Well I thought about it. The pointer (*v) does not change >>>> but the underlying cpumask variable is updated with the >>>> cpumask for the (supposedly) new node number. You can see >>>> that in this code snippet from kernel/sched.c: >>>> >>>> for (i = 1; i < SD_NODES_PER_DOMAIN; i++) { >>>> int next_node = find_next_best_node(node, &used_nodes); >>>> >>>> node_to_cpumask_ptr_next(nodemask, next_node); >>>> cpus_or(*span, *span, *nodemask); >>>> } >>>> >>>> In the optimized (x86_64) case, the pointer is simply modified >>>> to point to the new node_to_cpumask_map[node] entry. It remains >>>> a pointer to a const value. >>>> >>>> But the non-optimized version replaces the const cpumask value >>>> with the new cpumask value. Isn't this breaking the const >>>> attribute? >>> No, I think the pointer really should be const. This doesn't guarantee >>> that the value doesn't change behind our backs, it only prevents us >>> from modifying it ourselves. >>> >>> >>> Vegard >>> >> Is this what you had in mind: >> >> >> --- linux-2.6.tip.orig/include/asm-generic/topology.h >> +++ linux-2.6.tip/include/asm-generic/topology.h >> @@ -60,7 +60,7 @@ >> #ifndef node_to_cpumask_ptr >> >> #define node_to_cpumask_ptr(v, node) \ >> - cpumask_t _##v = node_to_cpumask(node), *v = &_##v >> + const cpumask_t _##v = node_to_cpumask(node), *v = &_##v >> >> #define node_to_cpumask_ptr_next(v, node) \ >> _##v = node_to_cpumask(node) >> >> >> (It's taking a while as now I need to do some cross-compile testing.) > > Actually, no. > > We don't want the _##v to be const, do we? What do you think about > this? (Watch out for whitespace munges) > > diff --git a/include/asm-generic/topology.h b/include/asm-generic/topology.h > index a6aea79..56957f2 100644 > --- a/include/asm-generic/topology.h > +++ b/include/asm-generic/topology.h > @@ -60,7 +60,8 @@ > #ifndef node_to_cpumask_ptr > > #define node_to_cpumask_ptr(v, node) > - cpumask_t _##v = node_to_cpumask(node), *v = &_##v > + cpumask_t _##v = node_to_cpumask(node); \ > + const cpumask_t *v = &_##v; > > #define node_to_cpumask_ptr_next(v, node) \ > _##v = node_to_cpumask(node) > > > Vegard >
Thanks. That was my alternative though I was hoping to confirm that the compiler detected the overwrite by node_to_cpumask_ptr_next(). Unfortunately every non-x86 cross-compile that I have for a machine that has NUMA is failing in some other way.
I'll resubmit with that change.
Mike
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