linux-kernel - Re: [PATCH 2/2] x86 : relocate uninitialized variable in init DATA section into init BSS section

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Date:	Fri, 22 Feb 2008 10:38:00 +0800
From:	"Huang, Ying" <ying.huang@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Ingo Molnar <mingo@...hat.com>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, Andi Kleen <ak@...e.de>,
	Ian Campbell <ijc@...lion.org.uk>,
	Matt Mackall <mpm@...enic.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86 : relocate uninitialized variable in init DATA
	section into init BSS section

On Thu, 2008-02-21 at 10:53 +0100, Ingo Molnar wrote:
> * Huang, Ying <ying.huang@...el.com> wrote:
> 
> > > > -int __initdata early_ioremap_debug;
> > > > +int __initbss early_ioremap_debug;
> > > 
> > > will we get some sort of build error if we accidentally do:
> > > 
> > >    int __initbss early_ioremap_debug = 1;
> > > 
> > > ?
> > 
> > I tested it just now, and there is no build error.
> 
> well, that's bad. We'd silently ignore the " = 1" and boot up with that 
> value at 0, right? At minimum we need some really prominent build-time 
> _errors_ (i.e. aborted builds) if this ever happens. But ideally, 
> shouldnt this whole thing be done at link time? Couldnt the linker sort 
> the variables that are zero initialized into the right section, and move 
> this constant maintenance pressure off the programmer's shoulder?

I think another method is to add a new attribute into GCC to prepend or
append something to section name instead of just to replace it, like the
example as follow:

#define __initdata  __attribute__((section_append(".init")))

int __initdata early_ioremap_debug_data = 1;
int __initdata early_ioremap_debug_bss;

The GCC can deduce the section (.data or .bss) of global variable based
on whether it is initialized. That is, without attribute,
early_ioremap_debug_data will be in ".data", early_ioremap_debug_bss
will be in ".bss". And with section_append attribute,
early_ioremap_debug_data will be in ".data.init" and
early_ioremap_debug_bss will be in ".bss.init".

Best Regards,
Huang Ying

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