How can insmod fail with kernel module is already loaded, even is lsmod does not contain that module?

Question

I'm making a minimal Linux distro with buildroot to learn kernel module development.

If I call a minimal module hello.ko (or almost any other name I've tried), everything works fine.

However, if I use the exact same code, but call the module workqueue.ko, insmod workqueue.ko fails, dmesg contains:

workqueue: module is already loaded

and insmod outputs to stderr:

insmod: can't insert 'workqueue.ko': invalid argument

Both lsmod and cat /proc/modules are empty.

This is the exact repo that produced the problem.

For reference the module code is:

#include <linux/module.h>
#include <linux/kernel.h>

MODULE_LICENSE("GPL");

int init_module(void)
{
    printk(KERN_INFO "hello init\n");
    return 0;
}

void cleanup_module(void)
{
    printk(KERN_INFO "hello cleanup\n");
}

and the kernel version (default implied by buildroot) is 4.9.

score 2 · Answer 1 · answered May 14 '17 at 11:40

2

$ uname -a
Linux alan-laptop 4.10.14-200.fc25.x86_64 #1 SMP Wed May 3 22:52:30 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
$ ls /sys/module/workqueue/
parameters  uevent
$ ls /sys/module/workqueue/parameters/
debug_force_rr_cpu  disable_numa  power_efficient

It's a builtin already. Maybe to provide a namespace for the parameters that effect kernel workqueue behaviour.

answered May 14 '17 at 11:40

sourcejedi

50,249

Hmmm, can you find where in the source that gets created? I could not find any settings in .config. Why not just use other /sys files that don't conflict with module names? – Ciro Santilli OurBigBook.com May 14 '17 at 11:45
1

@CiroSantilli709大抓捕六四事件法轮功 You know how to define a module, you can just use git grep to search for a definition of the module workqueue. – sourcejedi May 14 '17 at 12:10
@CiroSantilli包子露宪六四事件法轮功 that's not what the link says. It is /sys/module/ that shows builtins like this "workqueue". They don't show up in /proc/modules. – sourcejedi May 15 '18 at 17:46
Yes, you are right, I copied wrong. – Ciro Santilli OurBigBook.com May 15 '18 at 19:21
fun fact: some things under /sys/module are not necessarily moduels it seems https://unix.stackexchange.com/questions/225706/are-modules-listed-under-sys-module-all-the-loaded-modules/225746#225746 – Ciro Santilli OurBigBook.com May 15 '18 at 19:22

How can insmod fail with kernel module is already loaded, even is lsmod does not contain that module?

1 Answers1

Linked