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

Frans Pop
2009-03-09T15:05:16Z

On Sunday 08 March 2009, Frans Pop wrote:
> Kernels after 2.6.27 fail to boot in the Hercules S/390 emulator, quite
> early in the boot process:
[...]
> 0.141419! Initializing cgroup subsys ns
> 0.141605! Initializing cgroup subsys cpuacct
> 0.142009! Initializing cgroup subsys devices
> 0.180403! cpu: 2 configured CPUs, 0 standby CPUs
>
> I've bisected this to the following commit:
> commit 5cd1c9c5cf30d4b33df3d3f74d8142f278d536b7
> Author: Roman Zippel <zippel@linux-m68k.org>
> Date: Mon Sep 22 14:42:43 2008 -0700
> timekeeping: fix rounding problem during clock update

After staring at this commit for a while I decided to try the following
patch:
--- a/kernel/time/timekeeping.c
+++ b/kernel/time/timekeeping.c
@@ -547,5 +547,11 @@ void update_wall_time(void)
* add the remainder to the error difference.
*/
xtime.tv_nsec = ((s64)clock->xtime_nsec >> clock->shift) + 1;
+ if (unlikely(clock->xtime_nsec < ((s64)xtime.tv_nsec << clock->shift))) {
+ printk("Negative result: %llu - %lld\n",
+ (unsigned long long)clock->xtime_nsec,
+ (long long)((s64)xtime.tv_nsec << clock->shift));
+ BUG_ON(1);
+ }
clock->xtime_nsec -= (s64)xtime.tv_nsec << clock->shift;
clock->error += clock->xtime_nsec << (NTP_SCALE_SHIFT - clock->shift);

And that resulted in:
0.004175! Negative result: 166039808000 - 166039808256

So we're trying to stuff a negative value into an unsigned field, and I
keep seeing on these lists that's a bad idea.

I'll leave it up to you how we get there, but could it possibly be the
same kind of thing as corrected earlier today in Heiko Carsten's patch:
http://lkml.org/lkml/2009/3/9/112 ?

Cheers,
FJP