# insmod: ERROR: could not insert module *.ko: Unknown symbol in module
Date: 2019-06-17


# 问题：

         在实际编译运行  [什么是RCU ：API](https://garlicspace.com/2019/06/16/什么是rcu-%ef%bc%9aapi/)      中的例子 **[rcu\_example](https://github.com/jinb-park/rcu_example)**时报错

```
[root@centosgpt rcu]# insmod list_rcu.ko
insmod: ERROR: could not insert module list_rcu.ko: Unknown symbol in module
[root@centosgpt rcu]# dmesg|tail
[13084.720516] list_rcu: Unknown symbol synchronize_rcu (err -2)
[13084.720561] list_rcu: Unknown symbol call_rcu (err -2)
```

# 原因：

       在写代码的时候漏掉了原来例子中的  MODULE\_LICENSE("GPL");

       \_\_class\_create函数仅为 GPL模块导出（标记为使用 EXPORT\_SYMBOL\_GPL导出）的模块， 需要使用MODULE\_LICENSE("GPL")，才能使用相关功能。

```
/kernel/rcu/tree.c
...
void synchronize_rcu(void)
{
	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
			 lock_is_held(&rcu_lock_map) ||
			 lock_is_held(&rcu_sched_lock_map),
			 "Illegal synchronize_rcu() in RCU read-side critical section");
	if (rcu_blocking_is_gp())
		return;
	if (rcu_gp_is_expedited())
		synchronize_rcu_expedited();
	else
		wait_rcu_gp(call_rcu);
}
EXPORT_SYMBOL_GPL(synchronize_rcu);
...
void call_rcu(struct rcu_head *head, rcu_callback_t func)
{
	__call_rcu(head, func, -1, 0);
}
EXPORT_SYMBOL_GPL(call_rcu);
```

# 参考：

[How to Resolve – insmod: ERROR: could not insert module hello.ko: Unknown symbol in module – unknown symbol class\_unregister](https://www.lynxbee.com/how-to-resolve-insmod-error-could-not-insert-module-hello-ko-unknown-symbol-in-module-unknown-symbol-class_unregister/)

[unknown-symbol-class-create-err-0](https://stackoverflow.com/questions/29578931/unknown-symbol-class-create-err-0)