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

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Date:	Wed, 30 Apr 2008 05:49:46 -0700
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	"David Miller" <davem@...emloft.net>, <trini@...nel.crashing.org>
Cc:	<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

 

>-----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?

>Also, we have a macro for marking things weak "__weak" which should
>be used here.

Yes. Will change that.

Thanks,
Venki
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