Re: [patch] CFS scheduler, -v8

[Esben Nielsen <nielsen.esben@googlemail.com>]

On Sat, 5 May 2007, Linus Torvalds wrote:

> 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.

What is (long)(a-b) ? I have tried to look it up in the C99 standeard but 
I can't find it. Maybe it is in the referred LIA-1 standeard, which I 
can't find with google.

I think the best would be to use "a-b > ULONG_MAX/2" when you mean "a<b" 
as that should be completely portable.

According to C99 Appendix H2.2 
(http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf) an 
implementation can choose to do modulo signed integers as it is 
mandatory for unsigned integers. If an implementation have choosen 
to do that it must be a bug to to do the "a-b < 0" -> "a<b" optimization.

I have never experienced a compiler/architecture combination _not_ doing 
wrapped signed integers.

Esben


> 		Linus
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