内核通知链

linux内核提供通知链机制，用来提供一种异步通知机制。实现原理是将一些异步事件的回调函数链接到一个链表头上，在某种特定情况下，通过遍历该链表以调用所有注册到该链表上的事件回调函数。

通知链类型

linux内核定义了以下四种通知链：

Atomic通知链：在中断/原子操作上下文中调用通知回调。通知块回调函数不可以阻塞。

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struct atomic_notifier_head {
	spinlock_t lock;
	struct notifier_block __rcu *head;
};

blocking通知链：在进程上下文中的通知，允许通知块阻塞。

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struct blocking_notifier_head {
	struct rw_semaphore rwsem;
	struct notifier_block __rcu *head;
};

raw通知：通知块回调不做任何限制，所有的锁和保护由调用者提供。

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struct raw_notifier_head {
	struct notifier_block __rcu *head;
};

SRCU（Sleepable Read-Copy Update）通知：是一种Blocking通知链的变体，其限制和blocking通知链相同。用Sleepable Read-Copy Update代替了阻塞通知链中的读写信号量。具有开销小的优势，常用于通知块需要频繁调用，并且通知块基本不会删除的场景。

struct srcu_notifier_head {
struct mutex mutex;
struct srcu_struct srcu;
struct notifier_block __rcu *head;
};

通知链上的实例

Linux内核以一个notifier_block结构定义一个通知块：

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struct notifier_block {
	int (*notifier_call)(struct notifier_block *, unsigned long, void *);
	struct notifier_block __rcu *next;
	int priority;
};

其中：

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notifier_call是该通知实例的回调函数。
next指向该通知块所属通知链的下一个通知块。
priority表示该通知实例的优先级。

通知链定义和初始化接口

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/* srcu_notifier_heads must be initialized and cleaned up dynamically */
extern void srcu_init_notifier_head(struct srcu_notifier_head *nh);
#define srcu_cleanup_notifier_head(name)	\
		cleanup_srcu_struct(&(name)->srcu);

#define ATOMIC_NOTIFIER_INIT(name) {				\
		.lock = __SPIN_LOCK_UNLOCKED(name.lock),	\
		.head = NULL }
#define BLOCKING_NOTIFIER_INIT(name) {				\
		.rwsem = __RWSEM_INITIALIZER((name).rwsem),	\
		.head = NULL }
#define RAW_NOTIFIER_INIT(name)	{				\
		.head = NULL }
/* srcu_notifier_heads cannot be initialized statically */

#define ATOMIC_NOTIFIER_HEAD(name)				\
	struct atomic_notifier_head name =			\
		ATOMIC_NOTIFIER_INIT(name)
#define BLOCKING_NOTIFIER_HEAD(name)				\
	struct blocking_notifier_head name =			\
		BLOCKING_NOTIFIER_INIT(name)
#define RAW_NOTIFIER_HEAD(name)					\
	struct raw_notifier_head name =				\
		RAW_NOTIFIER_INIT(name)

通知链注册和取消注册接口

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extern int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
		struct notifier_block *nb);
extern int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
		struct notifier_block *nb);
extern int raw_notifier_chain_register(struct raw_notifier_head *nh,
		struct notifier_block *nb);
extern int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
		struct notifier_block *nb);

extern int blocking_notifier_chain_cond_register(
		struct blocking_notifier_head *nh,
		struct notifier_block *nb);

extern int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
		struct notifier_block *nb);
extern int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
		struct notifier_block *nb);
extern int raw_notifier_chain_unregister(struct raw_notifier_head *nh,
		struct notifier_block *nb);
extern int srcu_notifier_chain_unregister(struct srcu_notifier_head *nh,
		struct notifier_block *nb);

通知链执行实例接口

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extern int atomic_notifier_call_chain(struct atomic_notifier_head *nh,
		unsigned long val, void *v);
extern int __atomic_notifier_call_chain(struct atomic_notifier_head *nh,
	unsigned long val, void *v, int nr_to_call, int *nr_calls);
extern int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
		unsigned long val, void *v);
extern int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
	unsigned long val, void *v, int nr_to_call, int *nr_calls);
extern int raw_notifier_call_chain(struct raw_notifier_head *nh,
		unsigned long val, void *v);
extern int __raw_notifier_call_chain(struct raw_notifier_head *nh,
	unsigned long val, void *v, int nr_to_call, int *nr_calls);
extern int srcu_notifier_call_chain(struct srcu_notifier_head *nh,
		unsigned long val, void *v);
extern int __srcu_notifier_call_chain(struct srcu_notifier_head *nh,
	unsigned long val, void *v, int nr_to_call, int *nr_calls);

通知链的应用

以usb总线为例说明，usb子系统初始化的时候注册了usb总线，在总线注册的时候初始化了总线的通知链。

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int __bus_register(struct bus_type *bus, struct lock_class_key *key)
{
	/*
	 * ...
	 */
	BLOCKING_INIT_NOTIFIER_HEAD(&priv->bus_notifier);
	/*
	 * ...
	 */
}

接着向usb总线通知链上注册了通知实例usb_bus_nb：

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static struct notifier_block usb_bus_nb = {
	.notifier_call = usb_bus_notify,
};
...
/*usb_init*/
static int __init usb_init(void)
{
	/*
	 * ...
	 */
    retval = bus_register_notifier(&usb_bus_type, &usb_bus_nb);
	if (retval)
		goto bus_notifier_failed;
	/*
	 * ...
	 */
}

由其回调函数实现可知该通知实例的目的是在usb设备热插拔的时候在sysfs下创建和删除设备或接口文件。

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/*
 * Notifications of device and interface registration
 */
static int usb_bus_notify(struct notifier_block *nb, unsigned long action,
		void *data)
{
	struct device *dev = data;

	switch (action) {
	case BUS_NOTIFY_ADD_DEVICE:
		if (dev->type == &usb_device_type)
			(void) usb_create_sysfs_dev_files(to_usb_device(dev));
		else if (dev->type == &usb_if_device_type)
			usb_create_sysfs_intf_files(to_usb_interface(dev));
		break;

	case BUS_NOTIFY_DEL_DEVICE:
		if (dev->type == &usb_device_type)
			usb_remove_sysfs_dev_files(to_usb_device(dev));
		else if (dev->type == &usb_if_device_type)
			usb_remove_sysfs_intf_files(to_usb_interface(dev));
		break;
	}
	return 0;
}

当插入或拔出usb设备时，会通过usb_notify_add_device或者usb_notify_remove_device调用usb总线通知链上的回调函数。

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static int generic_probe(struct usb_device *udev)
{
	int err, c;

	/* Choose and set the configuration.  This registers the interfaces
	 * with the driver core and lets interface drivers bind to them.
	 */
	if (usb_device_is_owned(udev))
		;		/* Don't configure if the device is owned */
	else if (udev->authorized == 0)
		dev_err(&udev->dev, "Device is not authorized for usage\n");
	else {
		c = usb_choose_configuration(udev);
		if (c >= 0) {
			err = usb_set_configuration(udev, c);
			if (err) {
				dev_err(&udev->dev, "can't set config #%d, error %d\n",
					c, err);
				/* This need not be fatal.  The user can try to
				 * set other configurations. */
			}
		}
	}
	/* USB device state == configured ... usable */
	usb_notify_add_device(udev);

	return 0;
}

上面代码中usb_notify_add_device函数实现如下：

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void usb_notify_add_device(struct usb_device *udev)
{
	blocking_notifier_call_chain(&usb_notifier_list, USB_DEVICE_ADD, udev);
}