linux-kernel - huge gcc 4.1.{0,1} _

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Date:	Fri, 2 May 2008 00:56:33 +0300
From:	Adrian Bunk <bunk@...nel.org>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	David Miller <davem@...emloft.net>, trini@...nel.crashing.org,
	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>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: huge gcc 4.1.{0,1} __weak problem

On Wed, Apr 30, 2008 at 05:49:46AM -0700, Pallipadi, Venkatesh wrote:
>  
> >-----Original Message----- From: David Miller 
> >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.
>...

A workaround here is the wrong solution since this isn't the only place 
that suffers from this issue.

We currently give a #warning for 4.1.0.
But not for 4.1.1.
(Accordingto the bug >= 4.1.2 is fixed.)

And a #warning is not enough.

The huge problem is that "empty __weak function in the same file and 
non-weak arch function" has recently become a common pattern with 
several new usages added during this merge window alone.

And the breakages can be very subtle runtime breakages.

I see only the following choices:
- remove __weak and replace all current usages
- move all __weak functions into own files, and ensure that also happens
  for future usages
- #error for gcc 4.1.{0,1}

> Thanks,
> Venki

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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