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SubjectRe: for_each_cpu() is buggy for UP kernel?
On Wed, 9 May 2018, Andrew Morton wrote:

> On Wed, 9 May 2018 06:24:16 +0000 Dexuan Cui <decui@microsoft.com> wrote:
>
> > 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?
>
> I think so, yes. That might reveal new peculiarities, but such is life.
>
> I guess we should use bitmap_empty() rather than open-coding it.

Agreed.

FWIW, this had been discussed before, but there was no real conclusion:

https://lkml.kernel.org/r/alpine.DEB.2.20.1709161850010.2105@nanos

Thanks,

tglx

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