Re: arm: TI BeagleBoard X15 : Unable to handle kernel NULL pointer dereference at virtual address 00000369 - Internal error: Oops: 5 [#1] SMP ARM

[Naresh Kamboju]

Hi Arnd,

On Thu, 10 Nov 2022 at 03:33, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>
> On Wed, Nov 9, 2022, at 13:57, Arnd Bergmann wrote:
> >
> > One thing that sticks out is the print_constraints_debug() function
> > in the regulator framework, which uses a larger-than-average stack
> > to hold a string buffer, and then calls into the low-level
> > driver to get the actual data (regulator_get_voltage_rdev,
> > _regulator_is_enabled). Splitting the device access out into a
> > different function from the string handling might reduce the
> > stack usage enough to stay just under the 8KB limit, though it's
> > probably not a complete fix. I added the regulator maintainers
> > to Cc for thoughts on this.
>
> I checked the stack usage for each of the 147 functions in the
> backtrace, and as I was guessing print_constraints_debug() is
> the largest, but it's still only 168 bytes, and everything else
> is smaller, so no point hacking this.
>
> 168     print_constraints_debug
> 96      timekeeping_advance
> 64      set_machine_constraints
> 64      of_i2c_register_device
> 56      of_platform_bus_create
> 48      schedule_timeout
>
> One more idea I had is the unwinder: since this kernel is built
> with the frame-pointer unwinder, I think the stack usage per
> function is going to be slightly larger than with the arm unwinder.
>
> Naresh, how hard is it to reproduce this bug intentionally?
> Can you try if it still happens if you change the .config to
> use these:?
>
> # CONFIG_FUNCTION_GRAPH_TRACER is not set
> # CONFIG_UNWINDER_FRAME_POINTER is not set
> CONFIG_UNWINDER_ARM=y

I have done this experiment and reported crash not reproduced
after eight rounds of testing [1].

https://lkft.validation.linaro.org/scheduler/job/5835922#L1993

>
>        Arnd

- Naresh