Date: Wed, 7 May 2003 13:38:56 -0700
From: "Randy.Dunlap" <rddunlap@osdl.org>
To: Jeff Garzik <jgarzik@pobox.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: top stack (l)users for 2.5.69
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On Wed, 07 May 2003 15:17:43 -0400 Jeff Garzik <jgarzik@pobox.com> wrote:

| Linus Torvalds wrote:
| > In article <Pine.LNX.4.53.0305070933450.11740@chaos>,
| > Richard B. Johnson <root@chaos.analogic.com> wrote:
| > 
| >>You know (I hope) that allocating stuff on the stack is not
| >>"bad". 
| > 
| > 
| > Allocating stuff on the stack _is_ bad if you allocate more than a few
| > hundred bytes. That's _especially_ true deep down in the call-sequence,
| > ie in device drivers, low-level filesystems etc.
| > 
| > The kernel stack is a very limited resource, with no protection from
| > overflow. Being lazy and using automatic variables is a BAD BAD thing,
| > even if it's syntactically easy and generates good code.
| 
| 
| Note that the problem is exacerbated if you have a bunch of disjoint 
| stack scopes.  For that case, gcc will take the _sum_ of the stacks and 
| not the union.  rth was kind enough to file gcc PR 9997 on this problem.

Glad to hear that.
 
| It is turning out to be fairly common problem in the various drivers' 
| ioctl handlers.  Kernel hackers (myself included) often create automatic 
| variables for each case in a C switch statement.  (and now I'm having to 
| go back and fix that :))

I've written a few of the stack reduction patches.  Lots of ioctl functions
need work, so gcc handling it better would be good to have.

I have mostly used kmalloc/kfree, but using automatic variables is certainly
cleaner to write (code).  One of the patches that I did just made each ioctl
cmd call a separate function, and then each separate function was able to use
automatic variables on the stack instead of kmalloc/kfree.  I prefer this
method when it's feasible (and until gcc can handle these cases).

--
~Randy
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