Re: RFC: radix tree safety

[Andrew Morton <akpm@osdl.org>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
> Jonathan Corbet wrote:
> 
> >I've been digging through the radix tree code, and I noticed that the
> >tag functions have an interesting limitation.  The tag is given as an
> >integer value, but, in reality, the only values that work are zero and
> >one.  Anything else will return random results or (when setting tags)
> >corrupt unrelated memory.

Various people at various times have added additional tags.  reiser4...

> >The number of radix tree users is small, so it's not hard to confirm
> >that all tag values currently in use are legal.  But the interface would
> >seem to invite mistakes.
> >
> >The following patch puts in checks for out-of-range tag values.  I've
> >elected to have the relevant call fail; one could argue that it should
> >BUG instead.  Either seems better than silently doing weird stuff.  Not
> >2.6.16 material, obviously, but maybe suitable thereafter.
> >
> >
> 
> I'd agree if you make them BUG_ON()s.
> 
> Andrew Morton's kind of the radix-tree tags guy though... Andrew?

I don't really see the need - if someone goes and overindexes the data
structure's capacity then they have a bug and hopefully that'll turn up in
testing and will get fixed.

Or am I missing something obvious which makes radix-trees particularly
dangerous or subtle??