Re: Worst recursion in the kernel

From: Jörn Engel
Date: Thu Dec 04 2003 - 09:16:29 EST

On Wed, 3 December 2003 22:57:43 +0000, Russell King wrote:
> Yes, but the condition of the /data/ is such that it will not recurse.

Yes, so?

> A pure "can this function call that function" analysis ignoring the
> state of the data will say this will infinitely recuse. Include
> the data, and you'll find it has a very definite recursion limit.

That is what I do. My program takes hints saying "this recursion can
only loop n times". But I don't want to add semantic checking of the
source itself, so a human has to give the hint. Also, the human has
to uniquely hint at a single recursion.

If you accept this approach, there is no way to deal with multiple
linked recursions like this:

The human would have to say something like "the big recursion can only
happen five times, unless the short recursion from c to b happened.
In this case...". No thanks.

In fact, most recursions in the kernel are functions calling itself
again, there are just a few over several functions. So I honestly
wonder if recursion over multiple functions should be handled by my
program at all, or if I should just warn when seeing them.

There might be valid cases for two or three functions involved, so I
am not sure yet. But I sure as hell won't handle those cases before
seeing a valid use first and the one causing this thread sure isn't.


But this is not to say that the main benefit of Linux and other GPL
software is lower-cost. Control is the main benefit--cost is secondary.
-- Bruce Perens
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