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

[Rafael J. Wysocki]

On Mon, Feb 15, 2016 at 7:41 PM, Rafael J. Wysocki <rafael at kernel.org> 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.

Well, could there be a problem with sending an IPI to the same CPU
that's sending it?