Re: [BUG,2.6.28,s390] Fails to boot in Hercules S/390 emulator

john stultz
2009-03-12T00:47:57Z

On Wed, 2009-03-11 at 17:30 -0700, john stultz wrote:
> On Wed, 2009-03-11 at 17:03 +0100, Frans Pop wrote:
> > OK, I think I've gotten a lot further now.
> >
> > On Wednesday 11 March 2009, john stultz wrote:
> > > Also the negative conditional you add doesn't really make sense either,
> > > as we expect the xtime.tv_nsec << clock->shift to be larger then
> > > clock->xtime_nsec, as we've rounded it up by one. We then accumulate
> > > the negative difference between them into clock->error.
> >
> > I'm not at all fluent in casts, bit shifting and stuff, so it took a
> > while for the quarter to drop. But AFAICT what you're saying here is
> > exactly the problem.
> >
> > Indeed you do round xtime.tv_nsec up, so when you do
> > clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
> > or
> > clock->xtime_nsec = clock->xtime_nsec - ((s64)xtime.tv_nsec << clock->shift);
> > the second argument is always going to be bigger than the first, so you
> > always end up with a negative value.
> >
> > > Hmm.. Does the following explicit casting help?
> >
> > Even with the cast you're just papering over the issue that we're moving a
> > negative value into a field that is defined as unsigned:
> > include/linux/clocksource.h: u64 xtime_nsec;
>
> Probably agreed here, xtime_nsec probably should be converted to a s64
> as negative values are possible.
>
>
> However, Its unclear to me if my patch worked or not?
> Did you try it alone?

For a cleaner version, could you try the following, against 2.6.29-git
with no other modification?

thanks
-john

xtime_nsec is expected at times to be negative. Instead of trying to
handle all the shifting properly via casts, define it as a s64 instead
of a u64.

NOT FOR INCLUSION
Signed-off-by: John Stultz <johnstul@us.ibm.com>

diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h
index f88d32f..e217000 100644
--- a/include/linux/clocksource.h
+++ b/include/linux/clocksource.h
@@ -86,7 +86,7 @@ struct clocksource {
* more than one cache line.
*/
cycle_t cycle_last ____cacheline_aligned_in_smp;
- u64 xtime_nsec;
+ s64 xtime_nsec;
s64 error;
struct timespec raw_time;

diff --git a/kernel/time/timekeeping.c b/kernel/time/timekeeping.c
index 900f1b6..387be3c 100644
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -546,7 +546,7 @@ void update_wall_time(void)
/* store full nanoseconds into xtime after rounding it up and
* add the remainder to the error difference.
*/
- xtime.tv_nsec = ((s64)clock->xtime_nsec >> clock->shift) + 1;
+ xtime.tv_nsec = (clock->xtime_nsec >> clock->shift) + 1;
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
clock->error += clock->xtime_nsec << (NTP_SCALE_SHIFT - clock->shift);