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

[Andrew Morton <akpm@linux-foundation.org>]

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) :
_