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

[Andy Whitcroft <apw@shadowen.org>]

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
-
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