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

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Message-ID: <19f34abd0807081122p20c73cd9p7129d297ac36a3ad@mail.gmail.com>
Date:	Tue, 8 Jul 2008 20:22:57 +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 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

-- 
"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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