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SubjectRe: [PATCH 1/1] x86: Change _node_to_cpumask_ptr to return const ptr
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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