Messages in this thread Patch in this message | | | Date | Tue, 8 Jul 2008 23:21:50 +0200 | From | "Vegard Nossum" <> | Subject | Re: [PATCH 1/1] x86: Change _node_to_cpumask_ptr to return const ptr |
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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
-- "The animistic metaphor of the bug that maliciously sneaked in while the programmer was not looking is intellectually dishonest as it disguises that the error is the programmer's own creation." -- E. W. Dijkstra, EWD1036
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