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.
To unload a kernel module with recursive dependency, you need to first identify the modules currently in use using the
lsmod command. Then, determine the unloading order by unloading modules that other modules depend on last. Use the
rmmod command to remove the modules, and if encountering errors, identify and terminate the processes using the modules. If all else fails, you can try blacklisting the modules to prevent them from being loaded at startup.
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.
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
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
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
/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
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:
- Open the
/etc/modprobe.d/blacklist.conffile using a text editor with root privileges.
- Add the following lines to the file, replacing
drivernamewith the names of the modules you want to blacklist:
blacklist usbserial blacklist pl2303
- Save the file and reboot your system. The blacklisted modules should not be loaded at startup.
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.
A kernel module is a piece of object code that can be loaded into the running Linux kernel to add new functionality.
A recursive dependency occurs when a kernel module depends on itself either directly or indirectly through a series of other modules.
You can use the
lsmod command to list all loaded modules along with their dependencies.
Modules that other modules depend on should be unloaded last. You need to consider the dependencies and unload modules accordingly.
You can use the
rmmod command followed by the name of the module to remove it from the Linux kernel.
You need to identify the processes or applications that are using the module. Use the
lsof command to list open files and associated processes, then terminate or stop those processes before retrying the
You can edit the
/etc/modprobe.d/blacklist.conf file and add the names of the modules you want to blacklist. After saving the file, the blacklisted modules should not be loaded at startup.
It is important to proceed with caution when working with kernel modules and system files. Any mistake can lead to system instability or even a non-bootable system. Always refer to the documentation specific to your Linux distribution or seek assistance from a knowledgeable source if you are unsure about any step.