How to remove a failed kernel module

Question

This situation always bother me:

I wrote a kernel module, and sometimes it has a bug(dereference a NULL pointer). After I insmod hello.ko, it shows some kernel errors. Then I change the code, and try to remove the module and install it again. The question is: I don't know how to remove the kernel module.

$ rmmod hello

ERROR: module hello in use

$ rmmod -f hello

ERROR: removing hello: device or resource busy

I always reboot the machine in order to remove the module, which takes too long. Does anyone have a better solution for this? Thanks for any inputs.

Answer 1

Use a virtual machine.

Once you make a NULL dereference or other such mistake, you've put the kernel into an unknown state. Even if you did manage to remove the module (which is unlikely to be possible; a kernel OOPS kills the calling thread, so it'll never have a chance to reduce the reference count - the module will never be removable) there may still be corruption left behind, and your new, 'fixed' module is just as likely to be in trouble.

Much better to just use a fast-to-reboot virtual machine - perhaps with a snapshot, to make restoration even faster.