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SubjectRe: [PATCH] mm/memory.c: remove warning from an uninitialized spinlock. was: Re: 2.6.21-rc7-mm2
On Sat, Apr 28, 2007 at 07:57:40AM +0200, Borislav Petkov wrote:
> On Fri, Apr 27, 2007 at 05:22:30PM -0700, 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) :
> > _
>
> Yeah,
> I saw in other places that usually a NULL/0 is assigned to such a type of pointer.
> However, writing code which looks pretty silly just to shut up gcc is pretty
> senseless, IMHO. Isn't there such a tweak in gcc to say that this pointer is
> going to be assigned to later on, so don't issue a warning. Something like
>
> __attribute__ __address_will_be_overwritten_so_don't_bother_warning_me__?
>
> /me going to read gcc docs...

Sorry, no such thing in the docs to do

spinlock_t __attribute__((__uninitialized__)) *ptl;

in order to suppress warnings. But if function size is our concern here, even
shorter would be:

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 = 0;

pte = (mm == &init_mm) ?
pte_alloc_kernel(pmd, addr) :


--
Regards/Gruß,
Boris.
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