Re: [Linux PPC] Disable PREEMPT

[Shrikanth Hegde <sshegde@linux.ibm.com>]

Hi Michal,

On 9/17/26 3:44 PM, Michal Suchánek wrote:
> Hello,
> 
> On Wed, Sep 16, 2026 at 11:41:38PM +0530, Shrikanth Hegde wrote:
>> Hi Michal,
>>
>>>
>>> It seems easy to re-create.
>>> The key is running bcc/bpf program in parallel to kernel build.
>>>
>>> For example, these two in parallal, leads to stack corruption panic.
>>> Stacktrace does differ sometimes.
>>>
>>> ./funccount  rcu* -d 100
>>> make -j 32
>>>
>>> Boom. Will try to debug this further.
>>
>> A bit more debugging.
>>
>> 1. A few functions seems to crash, specially ones that call schedule.
>>     So picked schedule itself for tracing
>> 2. Used plain ftrace. No crash.
>> 3. Used kprobes. Boom. Crash appears. But this helped to avoid BPF JIT, or BCC part of it.
>> 4. Did some preempt count validation with annotations. Seems to be holding fine.
>> 5. Since unlock can call schedule while tracing schedule, there is possible looping.
>>     Need to see further why it happens to be crashing there. Maybe regs or paca
>>     handling needs a closer look.
>>
>> For now, below patch helps to keep the system stable under kprobes, bcc tools.
>> I will look further into see why exactly it is crashing and below one is going to
>> be the proper fix or not.
>>
>> Feel free to give it a try.
> 
> thanks for the patch.
> 
> While it is mangled by the e-mail client in different way compared to
> the previous patch it does seem to mitigate the issue.

Thanks for trying.

> 
> As it is unlikely the board where KVM lockup was originally observed is
> running kpbobes it will probably not address the original problem.
> 

Give it a try.

I am suspecting both lockup, stack corruptions are secondary symptoms here.
While recreate, I have different type of traces.

> Thanks
> 
> Michal
> 
>>
>> ---
>>   arch/powerpc/kernel/kprobes-ftrace.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
>> index f8208c027148..bcb1e8a8995d 100644
>> --- a/arch/powerpc/kernel/kprobes-ftrace.c
>> +++ b/arch/powerpc/kernel/kprobes-ftrace.c
>> @@ -63,7 +63,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
>>   		__this_cpu_write(current_kprobe, NULL);
>>   	}
>>   out:
>> +	preempt_disable_notrace();
>>   	ftrace_test_recursion_unlock(bit);
>> +	preempt_enable_no_resched_notrace();
>>   }
>>   NOKPROBE_SYMBOL(kprobe_ftrace_handler);
>> -- 
>> 2.52.0