lkml.org 
[lkml]   [2008]   [Jul]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
SubjectRe: [PATCH 1/1] x86: Change _node_to_cpumask_ptr to return const ptr
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.)

Thanks,
Mike


\
 
 \ /
  Last update: 2008-07-08 22:53    [W:0.046 / U:0.580 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site