Re: [Linux PPC] Disable PREEMPT

On Thu, Sep 17, 2026 at 03:58:42PM +0530, Shrikanth Hegde wrote:

Hi Michal,

On 9/17/26 3:44 PM, Michal Suchánek wrote:

quoted

Hello,

On Wed, Sep 16, 2026 at 11:41:38PM +0530, Shrikanth Hegde wrote:

quoted

Hi Michal,

quoted

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.

quoted

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.

Yes, it does work for the crash I see. However, we hijacked the report
of KVM locking up on FSL Cyrus+, and that is likely a different problem
still not addressed.

Thanks

Michal

quoted

Thanks

Michal

quoted

---
  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