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Date: Mon, 8 Feb 2010 21:28:39 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Andreas Mohr <andi@lisas.de>
cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
       John Stultz <johnstul@us.ibm.com>
Subject: Re: clocksource mutex deadlock, cat current_clocksource
 (2.6.33-rc6/7)
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On Mon, 8 Feb 2010, Andreas Mohr wrote:
> Hi,
> 
> On Mon, Feb 08, 2010 at 11:06:58AM +0100, Thomas Gleixner wrote:
> > EIP: 0060:[<c1045170>] EFLAGS: 00000082 CPU: 0
> > EIP is at timekeeping_forward_now+0x116/0x139
> > 
> > I don't see what might make the machine stuck here. Can you decode the
> > source line with "addr2line -e vmlinux c1045170" please ?
> 
> And the winner is:
> /usr/src/linux-2.6.33-rc7/include/linux/math64.h:91
> 
> static __always_inline u32
> __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
> {
>         u32 ret = 0;
> 
>         while (dividend >= divisor) {
>                 /* The following asm() prevents the compiler from
>                    optimising this loop into a modulo operation.  */
>                 asm("" : "+rm"(dividend));
> 
>                 dividend -= divisor;
>                 ret++;
>         }
> 
>         *remainder = dividend;
> 
>         return ret;
> }
> 
> 
> while ......
> 
> Do I see a divisor == 0 here?? ;)

The only function which is calling __iter_div_u64_rem() from
timekeeping_forward_now() is timespec_add_ns() which calls it with a
constant divisor:

static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
{
        a->tv_sec += __iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
        a->tv_nsec = ns;
}

There goes the theory :)

Which compiler version are you using ?

Can you please provide the disassembly of kernel/time/timekeeping.o ?

Thanks,

	tglx