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

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Date:	Mon, 9 Mar 2009 16:04:53 +0100
From:	Frans Pop <elendil@...net.nl>
To:	linux-s390@...r.kernel.org
Cc:	Roman Zippel <zippel@...ux-m68k.org>,
	John Stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [BUG,2.6.28,s390] Fails to boot in Hercules S/390 emulator

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@...ux-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
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