linux-kernel - Re: [PATCH] /dev/mem gcc weak function workaround

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Date:	Wed, 30 Apr 2008 13:15:08 -0700
From:	Tom Rini <trini@...nel.crashing.org>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	David Miller <davem@...emloft.net>, mingo@...e.hu,
	tglx@...utronix.de, hpa@...or.com, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: [PATCH] /dev/mem gcc weak function workaround

On Wed, Apr 30, 2008 at 05:49:46AM -0700, Pallipadi, Venkatesh wrote:
>  
> 
> >-----Original Message-----
> >From: David Miller [mailto:davem@...emloft.net] 
> >Sent: Tuesday, April 29, 2008 9:29 PM
> >To: Pallipadi, Venkatesh
> >Cc: mingo@...e.hu; tglx@...utronix.de; hpa@...or.com; 
> >akpm@...ux-foundation.org; linux-kernel@...r.kernel.org
> >Subject: Re: [PATCH] /dev/mem gcc weak function workaround
> >
> >From: Venki Pallipadi <venkatesh.pallipadi@...el.com>
> >Date: Tue, 29 Apr 2008 18:31:09 -0700
> >
> >> Some flavors of gcc 4.1.0 and 4.1.1 seems to have trouble 
> >understanding
> >> weak function definitions. Calls to function from the same 
> >file where it is
> >> defined as weak _may_ get inlined, even when there is a 
> >non-weak definition of
> >> the function elsewhere. I tried using attribute 'noinline' 
> >which does not
> >> seem to help either.
> >> 
> >> One workaround for this is to have weak functions defined in 
> >different
> >> file as below. Other possible way is to not use weak 
> >functions and go back
> >> to ifdef logic.
> >> 
> >> There are few other usages in kernel that seem to depend on 
> >weak (and noinline)
> >> working correctly, which can also potentially break and 
> >needs such workarounds.
> >> Example -
> >> mach_reboot_fixups() in arch/x86/kernel/reboot.c is one such 
> >call which
> >> is getting inlined with a flavor of gcc 4.1.1.
> >> 
> >> Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>
> >> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> >
> >This sounds like a bug.  And if gcc does multi-file compilation it
> >can in theory do the same mistake even if you move it to another
> >file.
> >
> >We need something more bulletproof here.
> >
> 
> The references here
> http://gcc.gnu.org/ml/gcc-bugs/2006-05/msg02801.html
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27781
> 
> seem to suggest that the bug is only with weak definition in the same
> file.
> So, having them in a different file should be good enough workaround
> here.
> 
> Tom: Comments?

Yes, that matches my recollection.  some versions of gcc 4.1.0 (and
possibly 4.1.1, it wouldn't be hard to check) could not handle having a
regular and weak function in the same file (I hit this trying to do
some abstraction or another in kgdb, using includes) but it was correct
if in separate files.  I assume blacklisting 4.1.0 is out of the
question :)

-- 
Tom Rini
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