Question

When I load this module:

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

MODULE_LICENSE("Dual BSD/GPL");

static int hello_init(void) {
  printk("<1> Hello world!\n");
  return 0;
}

static void hello_exit(void) {
  printk("<1> Bye, cruel world\n");
}


module_init(hello_init);
module_exit(hello_exit);

(From http://www.freesoftwaremagazine.com/articles/drivers_linux?page=0,2 )

The module get marked as [permanent] in lsmod and can’t be unloaded, on 2.6.39-02063904-generic (from the Ubuntu PPA). But it works fine on the default 2.6.38 kernel. (Both on Ubuntu 11.04 x86).

What has changed in 2.6.39? and what do I need to change in my code?

I was trying to isolate a more complicated problem when I ran into this issue.

EDIT:

Following a suggestion from an answer I edited the code to add __init and __exit (hello3.c):

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

MODULE_LICENSE("Dual BSD/GPL");

static int __init hello_init(void) {
  printk("<1> Hello world!\n");
  return 0;
}

static void __exit hello_exit(void) {
  printk("<1> Bye, cruel world\n");
}

module_init(hello_init);
module_exit(hello_exit);

The build output:

make -C /lib/modules/2.6.39-02063904-generic/build M=/home/douglas/kernelmod modules
make[1]: Entering directory `/usr/src/linux-headers-2.6.39-02063904-generic'
Building with KERNELRELEASE = 2.6.39-02063904-generic
  CC [M]  /home/douglas/kernelmod/hello3.o
  Building modules, stage 2.
Building with KERNELRELEASE = 2.6.39-02063904-generic
  MODPOST 8 modules
  CC      /home/douglas/kernelmod/hello3.mod.o
  LD [M]  /home/douglas/kernelmod/hello3.ko
make[1]: Leaving directory `/usr/src/linux-headers-2.6.39-02063904-generic'

EDIT2:

hello3.mod.c:

#include <linux/module.h>
#include <linux/vermagic.h>
#include <linux/compiler.h>

MODULE_INFO(vermagic, VERMAGIC_STRING);

struct module __this_module
__attribute__((section(".gnu.linkonce.this_module"))) = {
 .name = KBUILD_MODNAME,
 .init = init_module,
#ifdef CONFIG_MODULE_UNLOAD
 .exit = cleanup_module,
#endif
 .arch = MODULE_ARCH_INIT,
};

static const struct modversion_info ____versions[]
__used
__attribute__((section("__versions"))) = {
    { 0xbe4b3e92, "module_layout" },
    { 0xb4390f9a, "mcount" },
    { 0x5e3b3ab4, "printk" },
};

static const char __module_depends[]
__used
__attribute__((section(".modinfo"))) =
"depends=";


MODULE_INFO(srcversion, "D2A869459874C22AB265981");

Also

# grep CONFIG_MODULE_UNLOAD /boot/config-2.6.39-02063904-generic 
CONFIG_MODULE_UNLOAD=y

EDIT3:

More interestingly it doesn’t happen with a vanilla kernel I’ve compiled myself – that
loads and unloads modules fine.

EDIT4:

I installed the Oneiric beta 2 build on a VM, and that 3.0.0-11 kernel doesn’t have any problem either. So it appears to be limited to the Ubuntu Vanilla PPA kernels. That won’t be much fun to resolve.

Answer 1

So, after consultation with Canonical, I know what the problem is:

Ubuntu mainline builds are built with the Hardy tool chain, and the 11.04 and 11.10 tool chains are incompatible for building out-of-tree kernel modules.