Understanding Linux Network Internals

Registering with a Chain

When a kernel component is interested in the events of a given notification chain, it can register it with the general function notifier_chain_register. The kernel also provides a set of wrappers around notifier_chain_register, some of which are shown in Table 4-1.

Table 4-1 lists the main APIs and the associated wrappers used to register and unregister to the three chains inetaddr_chain , inet6addr_chain , and netdev_chain.

Table 4-1. Main functions and wrappers for a few chains

Operation	Function prototype
Registration	int notifier_chain_register(struct notifier_block **list, struct notifier_block *n)
	Wrappers
	inetaddr_chain	register_inetaddr_notifier
	inet6addr_chain	register_inet6addr_notifier
	netdev_chain	register_netdevice_notifier
Unregistration	int notifier_chain_unregister(struct notifier_block **nl, struct notifier_block *n)
	Wrappers
	inetaddr_chain	unregister_inetaddr_notifier
	inet6addr_chain	unregister_inet6addr_notifier
	netdev_chain	unregister_netdevice_notifier
Notification	int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)

For each chain, the notifier_block instances are inserted into a list, which is sorted by priority. Elements with the same priority are sorted based on insertion time: new ones go to the tail.

Accesses to the notification chains are protected by the notifier_lock lock. The use of a single lock for all the notification chains is not ...