Hi
When I insert external kernel module which is compiled out of tree I got
error:

root:/run# insmod ./my_module.ko
[  137.443823] my_module: Unknown symbol reboot_mode (err 0)

I investigated that I have attached in my my_module.c file:
#include <linux/reboot.h>
and in this file we have:
extern enum reboot_mode reboot_mode;
However reboot_mode symbol is not exported with macro EXPORT_SYMBOL_GPL.
Probably this is the reason why symbol is not seen in my module. Then why
the same issue doesn't occur here:
/arch/arm/kernel/setup.c
/arch/arm64/kernel/process.c
Those linux files also use global variable reboot_mode.