Re: huge gcc 4.1.{0,1} __weak problem

[Jeremy Fitzhardinge]

Adrian Bunk wrote:
> On Wed, Apr 30, 2008 at 05:49:46AM -0700, Pallipadi, Venkatesh wrote:
>   
> >  
> >     
> > > -----Original Message----- From: David Miller 
> > > From: Venki Pallipadi <venkatesh.pallipadi@xxxxxxxxx> 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@xxxxxxxxx>
> > > > Signed-off-by: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
> > > >         
> > > 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}
>   

- make __weak also include noinline.  I think that's sufficient (at 
least it was when I encountered a gcc bug with these symptoms last year 
or so).

   J
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