Re: [PATCH] mm/memory.c: remove warning from an uninitialized spinlock. was: Re: 2.6.21-rc7-mm2

Andy Whitcroft
2007-04-28T23:48:35Z

Andrew Morton wrote:
> On Thu, 26 Apr 2007 20:25:19 +0200
> Borislav Petkov <bbpetkov@yahoo.de> wrote:
>
>> Remove build warning mm/memory.c:1491: warning: 'ptl' may be used uninitialized in this function.
>> The spinlock pointer is assigned to null since it gets overwritten right away in
>> pte_alloc_map_lock().
>>
>> Signed-off-by: Borislav Petkov <bbpetkov@yahoo.de>
>> ---
>>
>> Index: linux-mm/mm/memory.c
>> ===================================================================
>> --- linux-mm.orig/mm/memory.c 2007-04-26 19:57:14.000000000 +0200
>> +++ linux-mm/mm/memory.c 2007-04-26 20:00:30.000000000 +0200
>> @@ -1488,7 +1488,7 @@
>> pte_t *pte;
>> int err;
>> struct page *pmd_page;
>> - spinlock_t *ptl;
>> + spinlock_t *ptl = NULL;
>>
>> pte = (mm == &init_mm) ?
>> pte_alloc_kernel(pmd, addr) :
>>
>
> yes, I've been staring unhappily at this for some time.
>
> Your change adds seven bytes of text to this function for no runtime
> benefit, just to fix a build-time warning. It's a general problem.
>
>
> Often we just leave the warning in place and curse gcc each time it flies
> past. Sometimes the code can be restructured in a sensible fashion to
> avoid the warning; often it cannot.
>
> But I don't think I want to put up with a warning coming out of core MM all
> the time so let's go with the following silliness which adds no additional
> runtime cost.
>
> --- a/mm/memory.c~add-apply_to_page_range-which-applies-a-function-to-a-pte-range-fix
> +++ a/mm/memory.c
> @@ -1455,7 +1455,7 @@ static int apply_to_pte_range(struct mm_
> pte_t *pte;
> int err;
> struct page *pmd_page;
> - spinlock_t *ptl;
> + spinlock_t *ptl = ptl; /* Suppress gcc warning */
>
> pte = (mm == &init_mm) ?
> pte_alloc_kernel(pmd, addr) :
> _
>

Perhaps we should have some kind definition helper.

#define suppress_unused(x) x = x

spinlock_t *suppress_unused(ptl);

Perhaps?

-apw