How To Unload a Kernel Module with Recursive Dependency

In the world of Linux, kernel modules are pieces of code that can be loaded and unloaded into the kernel upon demand. They extend the functionality of the kernel without the need to reboot the system. However, sometimes, you may need to unload a kernel module that has a recursive dependency. This article will guide you through the process.

Understanding Kernel Modules and Recursive Dependencies

A kernel module is a piece of object code that can be loaded into the running Linux kernel to add new functionality. Modules can depend on each other, meaning one module requires another to function properly. This is known as a module dependency. When a module depends on itself either directly or indirectly through a series of other modules, we call this a recursive dependency.

Identifying the Modules Currently in Use

Before you can unload a kernel module, you need to identify the modules currently in use. You can do this using the lsmod command. This command will list all loaded modules along with their dependencies.

lsmod

The output will show you the module name, memory size, the number of instances, and dependent modules.

Determining the Unloading Order

The order in which modules should be unloaded is crucial. Modules that other modules depend on should be unloaded last. For instance, if you need to unload the usbserial module and the pl2303 module, you should unload the usbserial module after the pl2303 module.

Unloading the Modules

You can unload the modules using the rmmod command. This command removes a module from the Linux kernel. Here’s how you can use it:

sudo rmmod pl2303
sudo rmmod usbserial

In the above commands, sudo gives you root privileges, rmmod is the command to remove a module, and pl2303 and usbserial are the names of the modules to be removed.

If you encounter an error message stating that the module is in use or cannot be removed, you need to identify the processes or applications that are using the modules.

Identifying the Processes Using the Modules

You can use the lsof command to list open files and the processes associated with them. For instance:

sudo lsof /dev/ttyUSB0

Replace /dev/ttyUSB0 with the appropriate device file for your driver. The lsof command will show you the processes that are using the device.

Terminating the Processes

Once you identify the processes or applications that are using the modules, you need to terminate or stop them. This may involve closing any applications that are accessing the device or terminating relevant processes.

After terminating the processes, retry unloading the modules using the rmmod command.

Blacklisting the Modules

If the above steps do not work, you can try blacklisting the modules to prevent them from being loaded at startup. However, be aware that blacklisting modules may have unintended consequences, so use this approach with caution.

To blacklist a module:

1. Open the /etc/modprobe.d/blacklist.conf file using a text editor with root privileges.
2. Add the following lines to the file, replacing drivername with the names of the modules you want to blacklist:

blacklist usbserial
blacklist pl2303

3. Save the file and reboot your system. The blacklisted modules should not be loaded at startup.

Conclusion

Unloading a kernel module with recursive dependency might seem like a daunting task, but by following the steps outlined in this article, you should be able to accomplish it with ease. Remember to always proceed with caution when dealing with kernel modules and system files. If you’re unsure about any step, consult the documentation specific to your Linux distribution or seek assistance from a knowledgeable source.