Re: buggy ia64_fls() ? (was Re: /dev/random problem on 2.6.12-rc1)

[Matt Mackall <mpm@selenic.com>]

On Fri, Apr 08, 2005 at 02:12:04PM +0200, Simon Derr wrote:
> I enabled the debug messages in random.c and I think I found the problem 
> lying in the IA64 version of fls().

Good catch.
 
> It turns out that the generic and IA64 versions of fls() disagree:
> 
> (output from a small test program)
> 
>      x   ia64_fls(x)      generic_fls(x)
> 
> i=-1, t=0, ia64: -65535 et generic:0
> i=0, t=1, ia64: 0 et generic:1
> i=1, t=2, ia64: 1 et generic:2
> i=2, t=4, ia64: 2 et generic:3
> i=3, t=8, ia64: 3 et generic:4

Well PPC at least sez:

/*
 * fls: find last (most-significant) bit set.
 * Note fls(0) = 0, fls(1) = 1, fls(0x80000000) = 32.
 */

And that agrees with the generic code (used by x86). So I think IA64
is probably wrong here indeed. It's amazing that the other users of
fls don't blow up spectacularly.

> I tried to fix it with an ia64 version that would give the same result as 
> the generic version, but the kernel did not boot, I guess some functions 
> rely on the ""broken"" ia64_fls() behaviour.
> 
> So I just changed fls() to use generic_fls() instead of ia64_fls().

If the "fixed" version didn't boot, how did the "alternate fixed"
version boot?

-- 
Mathematics is the supreme nostalgia of our time.