cannot insmoe workqueue: module already loaded, even though lsmod says otherwise

kernel_module/Makefile

@@ -1,4 +1,4 @@
-obj-m += $(addsuffix .o, $(notdir $(basename $(wildcard $(BR2_EXTERNAL_KERNEL_MODULE_PATH)/*.c))))
+obj-m += workqueue.o
 ccflags-y := -Wno-declaration-after-statement -std=gnu99
 
 .PHONY: all clean

kernel_module/workqueue.c

@@ -0,0 +1,13 @@
+#include <linux/module.h>
+#include <linux/kernel.h>
+
+int init_module(void)
+{
+	printk(KERN_INFO "hello init\n");
+	return 0;
+}
+
+void cleanup_module(void)
+{
+	printk(KERN_INFO "hello cleanup\n");
+}