Many parts of the kernel are well served as a module (dynamically-loadable parts of the kernel). Using the module_init() and module_exit() macros it is easy to write code without #ifdefs which can operate both as a module or built into the kernel.

The module_init() macro defines which function is to be called at module insertion time (if the file is compiled as a module), or at boot time: if the file is not compiled as a module the module_init() macro becomes equivalent to __initcall(), which through linker magic ensures that the function is called on boot.

The function can return a negative error number to cause module loading to fail (unfortunately, this has no effect if the module is compiled into the kernel). This function is called in user context with interrupts enabled, so it can sleep.