Crashes in arm qemu emulations due to 'cpufreq: governor: Replace timers with utilization ...'

[Rafael J. Wysocki]

On Mon, Feb 15, 2016 at 7:49 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 15/02/16 18:41, Rafael J. Wysocki wrote:
>> On Mon, Feb 15, 2016 at 6:05 PM, Guenter Roeck <linux at roeck-us.net> wrote:
>>> Rafael,
>>
>> Hi,
>>
>> Thanks for the report!
>>
>>> I see crashes in various arm qemu tests due to 'cpufreq: governor: Replace
>>> timers with utilization update callbacks' with next-20160215. An example
>>> crash log and bisect results are attached below.
>>>
>>> Please let me know if there is anything I can do to help tracking down
>>> the problem.
>>
>> It looks like we've uncovered some nastiness in the arch ARM code (see below).
>>
>> [cut]
>>
>>> [    1.340000] Unable to handle kernel NULL pointer dereference at virtual address 00000000
>>> [    1.340000] pgd = c0204000
>>> [    1.340000] [00000000] *pgd=00000000
>>> [    1.340000] Internal error: Oops: 80000005 [#1] SMP ARM
>>> [    1.340000] Modules linked in:
>>> [    1.340000] CPU: 0 PID: 1 Comm: swapper/0 Not tainted 4.5.0-rc4-next-20160215 #1
>>> [    1.340000] Hardware name: Generic OMAP3-GP (Flattened Device Tree)
>>> [    1.340000] task: cb060000 ti: cb05a000 task.ti: cb05a000
>>> [    1.340000] PC is at 0x0
>>> [    1.340000] LR is at arch_send_call_function_single_ipi+0x34/0x38
>>
>> Since this is ARM, arch_send_call_function_single_ipi() looks like this:
>>
>> void arch_send_call_function_single_ipi(int cpu)
>> {
>>          smp_cross_call(cpumask_of(cpu), IPI_CALL_FUNC_SINGLE);
>> }
>>
>> so I'm not sure how the NULL pointer deref is possible even.
>>
>> The only thing coming to mind would be that cpumask_of(cpu) triggers
>> this, but I'm not sure how exactly that can happen.
>>
>> I need help from somebody who knows how this low-level stuff works on ARM.
>
> Given that OMAP3 is a UP system, there is zero chance that it has
> registered the magic hook that delivers IPIs (its interrupt controller
> is not even capable of doing so).
>
> I don't really know the context, but IPIs on a UP system seem at best odd.

That would explain it, thanks.

So it looks like we should always use irq_work_queue() on UP even if
CONFIG_SMP is set, shouldn't we?

Thanks,
Rafael