Loading a kernel module at boot time is blocking?

Question

I am loading a kernel module at boot time, I added it to a config file in /etc/modules-load.d/, the module is loading correctly.

In my module I am using the wait_for_random_bytes() function from linux/random.h, so my module can have some delay in loading.

The modules are loaded sequentially? This module of mine can delay the loading of other modules? Thanks!

Answer 1

What the OS does ?

In my Debian (but i would bet that your CentOS just do the same), the module loading part of initialization is done by /etc/init.d/kmod.

Below is an extract of this script:

files=$(modules_files)
if [ "$files" ] ; then
  grep -h '^[^#]' $files |
  while read module args; do
    [ "$module" ] || continue
    load_module "$module" "$args"
  done
fi

Where:

• modules_files is a shell function that parses various file and directories (including /etc/modules-load.d) and built a list of modules to be loaded.

• load_module is a shell function that do the modprobe work + some logging if verbose flag is set.

So i would say that yes, modules are loaded sequentially and if one blocks, then it will block the other ones....

but ...

 What kernel does ?

When reading the source code of linux/modules.c we can see that:

• The syscall is probably implemented by the function load_module(). We can see that it performs a lot of stuff (initialization, memory allocation, sanity checks, signature checks, etc..) and it returns with return do_init_module(mod); (line 3927

• The do_init_module() function do at line 3574 the following operation and, if everything went ok, return 0.

    if (mod->init != NULL)
        ret = do_one_initcall(mod->init);
    if (ret < 0) {
        goto fail_free_freeinit;
    }

My conclusion is the syscall will return only when: 1. The module has been loaded in memory. 2. Its init() function has ran successfully.

So if your call to wait_for_random_bytes() is part of the init function of your module, then yes, it may block others modules loading.