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Date:	Tue, 8 Jul 2008 23:21:50 +0200
From:	"Vegard Nossum" <vegard.nossum@...il.com>
To:	"Mike Travis" <travis@....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

On Tue, Jul 8, 2008 at 10:51 PM, Mike Travis <travis@....com> wrote:
> 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.)

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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