The mutex interruptible and killable variants

As you have already learned, the mutex_lock_interruptible() API is used when the driver (or module) is willing to acknowledge any (user space) signal interrupting it (and returns -ERESTARTSYS to tell the kernel VFS layer to perform signal handling; the user space system call will fail with errno set to EINTR). An example can be found in the module handling code in the kernel, within the delete_module(2) system call (which rmmod(8) invokes):

// kernel/module.c
[ ... ]
SYSCALL_DEFINE2(delete_module, const char __user *, name_user,
        unsigned int, flags)
{
    struct module *mod;
    [ ... ]
    if (!capable(CAP_SYS_MODULE) || modules_disabled)
        return -EPERM;
    [ ... ]
    if (mutex_lock_interruptible(&module_mutex) != 0)
        return -EINTR;
    mod = find_module(name);
    [ ... ]
out:
    mutex_unlock(&module_mutex);
    return ret;
}

Notice how the API returns -EINTR on failure. (The SYSCALL_DEFINEn...