for_each_cpu() is buggy for UP kernel?

[Dexuan Cui <decui@microsoft.com>]

In include/linux/cpumask.h, for_each_cpu is defined like this for UP kernel (CONFIG_NR_CPUS=1):

#define for_each_cpu(cpu, mask)                 \
        for ((cpu) = 0; (cpu) < 1; (cpu)++, (void)mask)

Here 'mask' is ignored, but what if 'mask' contains 0 CPU? -- in this case, the for loop should not
run at all, but with the current code, we run the loop once with cpu==0.

I think I'm seeing a bug in my UP kernel that is caused by the buggy for_each_cpu():

in kernel/time/tick-broadcast.c: tick_handle_oneshot_broadcast(), tick_broadcast_oneshot_mask
contains 0 CPU, but due to the buggy for_each_cpu(), the variable 'next_event' is changed from
its default value KTIME_MAX to "next_event = td->evtdev->next_event"; as a result,
tick_handle_oneshot_broadcast () -> tick_broadcast_set_event() -> clockevents_program_event()
-> pit_next_event() is programming the PIT timer by accident, causing an interrupt storm of PIT
interrupts in some way: I'm seeing that the kernel is receiving ~8000 PIT interrupts per second for
1~5 minutes when the UP kernel boots, and it looks the kernel hangs, but in 1~5 minutes, finally
somehow the kernel can recover and boot up fine. But, occasionally, the kernel just hangs there
forever, receiving ~8000 PIT timers per second. 

With the below change in kernel/time/tick-broadcast.c, the interrupt storm will go away:

+#undef for_each_cpu
+#define for_each_cpu(cpu, mask)                        \
+       for ((cpu) = 0; (((cpu) < 1) && ((mask)[0].bits[0] & 1)); (cpu)++, (void)mask)

Should we fix the for_each_cpu() in include/linux/cpumask.h for UP?

Thanks,
-- Dexuan