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

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Message-ID: <20070428062516.GB4957@gollum.tnic>
Date:	Sat, 28 Apr 2007 08:25:17 +0200
From:	Borislav Petkov <bbpetkov@...oo.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org,
	Jeremy Fitzhardinge <jeremy@...source.com>
Subject: Re: [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@...oo.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@...oo.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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