Re: [patch] CFS scheduler, -v8

Linus Torvalds
2007-05-05T17:45:49Z

On Sat, 5 May 2007, Esben Nielsen wrote:
>
> I have been wondering why you use usigned for timers anyway. It is also like
> that in hrtimers. Why not use signed and avoid (almost) all worries about wrap
> around issues. The trick is that when all
> a < b
> is be replaced by
> a - b < 0
> the code will work on all 2-complement machines even if the (signed!) integers
> a and b wrap around.

No. BOTH of the above are buggy.

The C language definition doesn't allow signed integers to wrap (ie it's
undefined behaviour), so "a-b < 0" can be rewritten by the compiler as a
simple signed "a < b".

And the unsigned (or signed) "a < b" is just broken wrt any kind of
wrap-around (whether wrapping around zero or the sign bit).

So the _only_ valid way to handle timers is to
- either not allow wrapping at all (in which case "unsigned" is better,
since it is bigger)
- or use wrapping explicitly, and use unsigned arithmetic (which is
well-defined in C) and do something like "(long)(a-b) > 0".

Notice? The signed variant is basically _never_ correct.

Linus