Module dependencies

README.md

@@ -330,3 +330,6 @@ But TODO I don't think you can see where you are in the kernel source code and l
             1. [timer](kernel_module/timer.c)
             1. [work_from_work](kernel_module/work_from_work.c)
         1.  [irq](irq.c)
+        1.  Module dependencies
+            1. [dep.c](kernel_module/dep.c)
+            1. [dep2.c](kernel_module/dep2.c)

busybox_config_fragment

@@ -1,3 +1,4 @@
+CONFIG_DEPMOD=y
 CONFIG_MODINFO=y
 CONFIG_NC=y
 CONFIG_NC_EXTRA=y

kernel_module/dep.c

@@ -0,0 +1,73 @@
+/*
+Exports the lkmc_dep which dep2.ko uses.
+
+    insmod /dep.ko
+    # dmesg => 0
+    # dmesg => 0
+    # dmesg => ...
+    insmod /dep2.ko
+    # dmesg => 1
+    # dmesg => 2
+    # dmesg => ...
+    rmmod dep
+    # Fails because dep2 uses it.
+    rmmod dep2
+    # Dmesg stops incrementing.
+    rmmod dep
+
+sys visibility:
+
+    dmesg -n 1
+    insmod /dep.ko
+    insmod /dep2.ko
+    ls -l /sys/module/dep/holders
+    # => ../../dep2
+    cat refcnt
+    # => 1
+
+depmod:
+
+    grep dep "/lib/module/"*"/depmod"
+    # extra/dep2.ko: extra/dep.ko
+    # extra/dep.ko:
+    modprobe dep
+    # lsmod
+    # Both dep and dep2 were loaded.
+
+TODO: at what point does buildroot / busybox generate that file?
+*/
+
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+
+MODULE_LICENSE("GPL");
+
+int lkmc_dep = 0;
+EXPORT_SYMBOL(lkmc_dep);
+static struct task_struct *kthread;
+
+static int work_func(void *data)
+{
+	while (!kthread_should_stop()) {
+		printk(KERN_INFO "%d\n", lkmc_dep);
+		usleep_range(1000000, 1000001);
+	}
+	return 0;
+}
+
+static int myinit(void)
+{
+	kthread = kthread_create(work_func, NULL, "mykthread");
+	wake_up_process(kthread);
+	return 0;
+}
+
+static void myexit(void)
+{
+	kthread_stop(kthread);
+}
+
+module_init(myinit)
+module_exit(myexit)

kernel_module/dep2.c

@@ -0,0 +1,33 @@
+#include <linux/delay.h> /* usleep_range */
+#include <linux/kernel.h>
+#include <linux/kthread.h>
+#include <linux/module.h>
+
+MODULE_LICENSE("GPL");
+
+extern int lkmc_dep;
+static struct task_struct *kthread;
+
+static int work_func(void *data)
+{
+	while (!kthread_should_stop()) {
+		usleep_range(1000000, 1000001);
+		lkmc_dep++;
+	}
+	return 0;
+}
+
+static int myinit(void)
+{
+	kthread = kthread_create(work_func, NULL, "mykthread");
+	wake_up_process(kthread);
+	return 0;
+}
+
+static void myexit(void)
+{
+	kthread_stop(kthread);
+}
+
+module_init(myinit)
+module_exit(myexit)