Calling Linux Kernel methods from a Kernel Module

What is the right way of calling kernel functions in a C file from a kernel module in Linux?

I want to call exit_task_namespaces in linux/nsproxy.c from my first ever kernel module

I am doing this:

#include <linux/nsproxy.h>…static ssize_t device_read(struct file *flip, char *buffer, size_t len, loff_t *offset){    struct task_struct *task = current;…    exit_task_namespaces(task);…}

when I try to make I get the following error:

ERROR: "exit_task_namespaces" [/home/.../lkm_example.ko] undefined!make[2]: *** [scripts/Makefile.modpost:94: __modpost] Error 1make[1]: *** [Makefile:1673: modules] Error 2make[1]: Leaving directory '/usr/src/linux-headers-5.4.0-73-generic'make: *** [Makefile:3: all] Error 2

I can see that in the file /usr/src/linux-headers-5.4.0-73-generic/include/linux/nsproxy.h the method exists.

This is my Makefile:

obj-m += lkm_example.oall:    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules