How to put a check in the code to ensure the inter kernel module dependency - Linux Kernel?

Question

I have two modules. I want the modules to be interdependent while doing insmod or rmmod. Currently, my module2 is dependent on module1. If I insert module1 then module2, it works fine. On the other hand, the reverse of it doesn't work. This is logical in explanation. However, I want a neat code to avoid such dependencies. If I do insmod of mod2 then mod1 should automatically be insmod, Or some other decent way to tackle this issue. Here are my two modules.

static int multiplyMod1(int a, int b);
/** Once the symbol is exported, check in /proc/kallsyms **/
/** Call multiplyMod1 from other module **/
/** Ensure that this symbol doesn't exist somewhere **/
EXPORT_SYMBOL(multiplyMod1);

static int multiplyMod1(int a, int b)
{
  return a*b;
}

static int mod1_init(void)
{
    printk(KERN_ALERT "Hello mod1..Init\n");
    return 0;
}

static void mod1_exit(void)
{
    printk(KERN_ALERT "Goodbye..mod1\n");
}

module_init(mod1_init);
module_exit(mod1_exit);

The makefile -

obj-m += mod1.o

all:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules modules_install

clean:
    make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Here is my second module. This is dependent on the module 1.

/** Ensure that the mod1 is insmod so that multiplyMod1 symbol is available **/

extern int multiplyMod1(int,int);

static int mod2_init(void)
{
    printk(KERN_ALERT "Hello mod2..Init\r\n");
    printk(KERN_ALERT "The Multiplication result from Mod2 is..%d\r\n",multiplyMod1(49,7));
    return 0;
}

static void mod2_exit(void)
{
    printk(KERN_ALERT "Goodbye..mod2\r\n");
}

module_init(mod2_init);
module_exit(mod2_exit);

Answer 1

You can use request_module API in module mod2 in which you want to use exported symbol, this loads required module on demand if not loaded previously.

In module mod2,

static int __init module_two_init_module(void)
{
        int ret;
        const char *name;
        struct module * fmodule;
        pr_info("Module two started");
        name = "module_one";
        /* This will find module is loaded or not */
        fmodule = find_module(name); 
        if (fmodule == NULL) {
                ret = request_module(name);
                if (ret == 0) {
                        pr_info("Module one loaded");
                }
        }
        /* Do rest of code here */

Don't forget to increment module reference count using try_module_get()