[PATCH] powerpc/sysrq: Fix oops whem ppmu is not registered

[Madhavan Srinivasan]

On Tuesday 19 September 2017 03:30 PM, Michael Ellerman wrote:
> Ravi Bangoria <ravi.bangoria at linux.vnet.ibm.com> writes:
>
>> Kernel crashes if power pmu is not registered and user tries to dump
>> regs with 'echo p > /proc/sysrq-trigger'. Sample log:
>>
>>    Unable to handle kernel paging request for data at address 0x00000008
>>    Faulting instruction address: 0xc0000000000d52f0
>>
>>    NIP [c0000000000d52f0] perf_event_print_debug+0x10/0x230
>>    LR [c00000000058a938] sysrq_handle_showregs+0x38/0x50
>>    Call Trace:
>>     printk+0x38/0x4c (unreliable)
>>     __handle_sysrq+0xe4/0x270
>>     write_sysrq_trigger+0x64/0x80
>>     proc_reg_write+0x80/0xd0
>>     __vfs_write+0x40/0x200
>>     vfs_write+0xc8/0x240
>>     SyS_write+0x60/0x110
>>     system_call+0x58/0x6c
> Thanks.
>
> I added:
>
>    Fixes: 5f6d0380c640 ("powerpc/perf: Define perf_event_print_debug() to print PMU register values")
>
> But I don't think I'll Cc stable, I can't think why we'd ever *not* have
> a ppmu registered. How did you hit it?
mpe,

We hit this in a compact mode setup where the distro kernel did not have 
the
pmu support for the base pvr.

Maddy


> cheers
>