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Re: [PATCH] Discardable strings for init and exit sections

To: Ralf Baechle <ralf@linux-mips.org>
Subject: Re: [PATCH] Discardable strings for init and exit sections
From: Adrian Bunk <bunk@kernel.org>
Date: Fri, 12 Oct 2007 20:15:44 +0200
Cc: "Maciej W. Rozycki" <macro@linux-mips.org>, Franck Bui-Huu <vagabon.xyz@gmail.com>, linux-arch@vger.kernel.org, linux-mips@linux-mips.org, Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org
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References: <Pine.LNX.4.64N.0710121711120.21684@blysk.ds.pg.gda.pl> <20071012171938.GB6476@stusta.de> <20071012175209.GA1110@linux-mips.org>
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On Fri, Oct 12, 2007 at 06:52:10PM +0100, Ralf Baechle wrote:
> On Fri, Oct 12, 2007 at 07:19:38PM +0200, Adrian Bunk wrote:
> 
> > I have an objection against this approach:
> > 
> > Our __*init*/__*exit* annotations are already a constant source of bugs, 
> > and adding more pifalls (e.g. forgotten removal of _i()/_e() when a 
> > function is no longer __*init*/__*exit*) doesn't sound like a good plan.
> > 
> > Shouldn't it be possible to automatically determine where to put the 
> > strings? I don't know enough gcc/ld voodoo for being able to tell 
> > whether it is currently possible, and if yes how, but doing it 
> > automatically sounds like the only solution that wouldn't result in an
> > unmaintainable mess.
> 
> gcc tends to place data such as strings or jump tables generated from
> switches not into a place were it would be easily discardable.  The
> latter is the reason that on MIPS we can't discard __exit functions
> at all - a switch table in .rodata might be referencing discarded code
> in .exit.text which makes ld fail.  When I discussed this with some gcc
> people a while ago nobody really had a good suggestion to solve this.

- Most of the string annotations are (naturally) dev{init,exit}
  annotations, and bugs there are therefore in configurations that have
  only extremely low testing coverage during -rc.
- I'm counting 22 annotations in the driver Maciej converted as an
  example. When estimating the number of possible annotations based
  on the number of C files in the kernel I'm getting a six digit
  number.

No matter how hard it would be to teach gcc about it, when thinking of 
the amount of __*init*/__*exit* bugs we already have I simply can't 
imagine the string annotations as a maintainable solution.

>   Ralf

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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