linux-kernel - Re: [PATCH 1/1] x86: Change _node_to_cpumask

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Message-ID: <4873D33B.4040300@sgi.com>
Date:	Tue, 08 Jul 2008 13:51:07 -0700
From:	Mike Travis <travis@....com>
To:	Vegard Nossum <vegard.nossum@...il.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	mm-commits@...r.kernel.org, Yinghai Lu <yhlu.kernel@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [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@....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
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