lkml.org 
[lkml]   [2005]   [Apr]   [8]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: buggy ia64_fls() ? (was Re: /dev/random problem on 2.6.12-rc1)
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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

\
 
 \ /
  Last update: 2005-04-08 18:32    [W:0.047 / U:0.268 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site