linux-kernel - Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

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Date:	Wed, 27 Feb 2008 07:33:07 -0300
From:	Mauro Carvalho Chehab <mchehab@...radead.org>
To:	Marcin Slusarz <marcin.slusarz@...il.com>
Cc:	Jean Delvare <khali@...ux-fr.org>,
	LKML <linux-kernel@...r.kernel.org>, i2c@...sensors.org,
	video4linux-list@...hat.com
Subject: Re: [PATCH] video: limit stack usage of ir-kbd-i2c.c

On Wed, 27 Feb 2008 11:23:26 +0100
Marcin Slusarz <marcin.slusarz@...il.com> wrote:

> On Tue, Feb 26, 2008 at 11:23:20PM +0100, Jean Delvare wrote:
> > Hi Marcin,
> Hi
>  
> > On Tue, 26 Feb 2008 22:03:16 +0100, Marcin Slusarz wrote:
> > > Do you have an idea (or patch :D) how to solve this:
> > > 0x00000234 v4l_compat_translate_ioctl [v4l1-compat]:    1376
> > > ? That's on top of my make checkstack output
> > 
> > Random ideas (but I am in no way a specialist of this exercise):
> > 
> > * You could try moving the structures to the blocks where they are used,
> > in the case a given structure is used for only one ioctl. I'm not too
> > sure how gcc handles local variables declared inside blocks with
> > regards to stack reservation though. I thought it would work but my
> > experiments today seem to suggest it doesn't.
> That won't work. Variables at beginning of function take only ~600 bytes,
> so the rest must be from inner blocks and inlines (probably).
> 
> > * You can move the handling of some ioctls to dedicated functions, just
> > like I did in i2c-dev:
> > http://lists.lm-sensors.org/pipermail/i2c/2008-February/003010.html
> > However there is a risk that gcc will inline these functions (that's
> > what happened to me...) Not sure how to prevent gcc from inlining.
> There's "noinline" attribute in linux/compiler.h (compiler-gcc.h actually)
> for these situations.
>  
> > * You can allocate the structures dynamically, as you originally wanted
> > to do for ir-kbd-i2c. However this has a performance penalty and will
> > fragment the memory, so it's not ideal.
> > 
> > * If each ioctl uses only one of the structures, you may define a union
> > of all the structures. The size of the union will be the size of the
> > biggest structure, so you save a lot of space on the stack.
> Nice idea.
> 
> I'll try 2nd and 4th approaches.

The union will probably solve. This function is very complex, since it needs to
deal with almost all v4l1 v4l2 ioctls (about 80-90). Splitting into small
functions might help, but probably, gcc will create the functions as inline.


> 
> Marcin Slusarz



Cheers,
Mauro
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