1. 引言
Linux是单内核架构(monolithic kernel),大多数内核子系统和模块是相互独立的,它们被动态地加载或卸载,以使内核变得小巧和可扩展.然而,子系统或模块之间需要通信,或者说某个特定模块扑捉到的事件可能其它模块对此感兴趣,这就需要一种机制来满足子系统或模块之间交互的需求.
Linux使用通知链表来实现这一需求,它是一个简单的函数链表,当某件事件发生时,链表上的函数就会执行.这是一种发布-订阅(publish-subscribe)模式,当客户(订阅者)需要某个特定事件的通知时,会向主机(发布者)注册自己;接下来,只要感兴趣的事件一发生,主机便会通知客户.
讨论内核通知链之前,有必要注意以下几点:
- 通知链表只能够在内核的子系统之间使用,而不能够在内核与用户空间之间进行事件的通知.
- 通知链表是一个函数链表,链表上的每一个节点都注册了一个函数.当某个事情发生时,链表上所有节点对应的函数就会被执行.所以对于通知链表来说有一个通知方(主机)与一个接收方(订阅者).
- 在通知这个事件时所运行的函数由被通知方(订阅者通过回调函数)决定,实际上也即是被通知方注册(订阅者实现)了某个函数,在发生某个事件时这些函数就得到执行.
2. 数据结构
清单1. 原子通知链(Atomic notifier chains):
通知链元素的回调函数(当事件发生时要执行的函数)只能在中断上下文中运行,不允许阻塞
1 struct atomic_notifier_head {
2 spinlock_t lock; /*锁*/
3 struct notifier_block *head;
4 };
清单2. 可阻塞通知链(Blocking notifier chains):
阻塞通知头结构,增加了一个读写信号灯,通知链元素的回调函数在进程上下文中运行,允许阻塞
1 struct blocking_notifier_head {
2 struct rw_semaphore rwsem; /*读写控制信号量*/
3 struct notifier_block *head;
4 };
清单3. 原始通知链(Raw notifier chains):
原始通知头结构,就是一个通知块指针,对通知链元素的回调函数没有任何限制,所有锁和保护机制都由调用者维护
1 struct raw_notifier_head {
2 struct notifier_block *head;
3 };
清单4. SRCU 通知链(SRCU notifier chains):
可阻塞通知链的一种变体,(SRCU)Sleepable Read Copy Update 的链表通知,与block链表通知类似,不同在处理锁与保护上,SRCU在调用通知时的系统开销小,而从通知链表中去除通知调用的系统开销大,因此适合用在调用通知频繁,而移除调用通知少的情况中
1 struct srcu_notifier_head {
2 struct mutex mutex;
3 struct srcu_struct srcu;
4 struct notifier_block *head;
5 };
参看：http://geyingzhen.blog.163.com/blog/static/6558665520091165531675/
清单5. 基本的通知块结构(notifier_block):
通知链的核心结构
1 struct notifier_block {
2 int (*notifier_call)(struct notifier_block *, unsigned long, void *); /*回调函数*/
3 struct notifier_block *next; /*指向链表的下一个元素节点，这是一个单向链表*/
4 int priority; /*该块的优先级, 在链表中各个块是按此优先级值进
5 *行排序的, 值大的在链表前, 表明
6 *相应回调函数执行的顺序*/
7 };
其中,函数指针notifier_call注册了当某个事件发生时需要调用的函数;next指向下一个链表节点;priority设定链表节点的优先级;数值越大优先级越高,默认为0.因此,所有的通知链表节点组成了一个单链表,并以优先级(priority)排列.
3. 运行机制
通知链的运作机制包括两个角色:
- 被通知者(订阅者):对某一事件感兴趣一方,定义了当事件发生时,相应的处理函数,即回调函数.但需要事先将其注册到通知链中(被通知者注册的动作就是在通知链中增加一项).
- 通知者(主机):事件的通知者.当检测到某事件,或者本身产生事件时,通知所有对该事件感兴趣的一方事件发生.他定义了一个通知链,其中保存了每一个被通知者对事件的处理函数(回调函数).通知这个过程实际上就是遍历通知链中的每一项,然后调用相应的事件处理函数.
通知机制实现包括以下三个步骤:
- 通知者(主机)定义通知链;
- 被通知者(订阅者)向通知链中注册回调函数;
- 当事件发生时,通知者(主机)发出通知(执行通知链中所有元素的回调函数).
清单6. notifier_chain_register函数:
被通知者(订阅者)调用 notifier_chain_register函数注册回调函数,该函数按照优先级将回调函数加入到通知链中:
1 /*
2 * Notifier chain core routines. The exported routines below
3 * are layered on top of these, with appropriate locking added.
4 */
5 /*nl是链表头块的地址, n是要添加到该链表的通知块*/
6 static int notifier_chain_register(struct notifier_block **nl,
7 struct notifier_block *n)
8 {
9 while ((*nl) != NULL) { /*使用的是dummy header算法, 即使刚开始时链表为空也不用显示判断区分*/
10 if (n->priority > (*nl)->priority) /*判断优先权值, 优先权值越大位置越靠前*/
11 break;
12 nl = &((*nl)->next);
13 }
14 n->next = *nl; /*将节点n链接到链表nl中的合适位置*/
15 rcu_assign_pointer(*nl, n); /*使用rcu处理函数保证SMP下的安全性, 相当于加上锁再赋值*/
16 return 0;
17 }
清单7. notifier_chain_unregister函数:
注销回调函数则使用notifier_chain_unregister函数,即将回调函数从通知链中删除:
1 /*nl是链表头块的地址, n是要删除的通知块*/
2 static int notifier_chain_unregister(struct notifier_block **nl,
3 struct notifier_block *n)
4 {
5 while ((*nl) != NULL) {
6 if ((*nl) == n) {
7 rcu_assign_pointer(*nl, n->next); /* *nl=n->next的安全赋值操作,相当于将节点从链表断开*/
8 return 0;
9 }
10 nl = &((*nl)->next);
11 }
12 return -ENOENT;
13 }
清单8. notifier_call_chain函数:
通知者(主机)调用 notifier_call_chain函数通知事件的到达,这个函数会遍历通知链中所有的元素,然后依次调用每一个的回调函数(即完成通知动作):
1 /**
2 * notifier_call_chain - Informs the registered notifiers about an event.
3 * @nl: Pointer to head of the blocking notifier chain
4 * @val: Value passed unmodified to notifier function
5 * @v: Pointer passed unmodified to notifier function
6 * @nr_to_call: Number of notifier functions to be called. Don't care
7 * value of this parameter is -1.
8 * @nr_calls: Records the number of notifications sent. Don't care
9 * value of this field is NULL.
10 * @returns: notifier_call_chain returns the value returned by the
11 * last notifier function called.
12 */
13 static int __kprobes notifier_call_chain(struct notifier_block **nl,
14 unsigned long val, void *v,
15 int nr_to_call, int *nr_calls)
16 {
17 int ret = NOTIFY_DONE;
18 struct notifier_block *nb, *next_nb;
19
20 nb = rcu_dereference(*nl); /*安全地获取通知块指针*/
21
22 while (nb && nr_to_call) { /*链表循环*/
23 next_nb = rcu_dereference(nb->next); /*找下一个块*/
24
25 #ifdef CONFIG_DEBUG_NOTIFIERS
26 if (unlikely(!func_ptr_is_kernel_text(nb->notifier_call))) {
27 WARN(1, "Invalid notifier called!");
28 nb = next_nb;
29 continue;
30 }
31 #endif
32 ret = nb->notifier_call(nb, val, v); /*执行订阅者注册的回调函数，对此通知做出响应*/
33
34 if (nr_calls)
35 (*nr_calls)++;
36
37 if ((ret & NOTIFY_STOP_MASK) == NOTIFY_STOP_MASK) /*如果返回停止标志, 不执行后续结构*/
38 break;
39 nb = next_nb; /*循环执行，进入下一个节点*/
40 nr_to_call--;
41 }
42 return ret;
43 }
参数nl是通知链的头部,val表示事件类型,v用来指向通知链上的函数执行时需要用到的参数,一般不同的通知链,参数类型也不一样,例如当通知一个网卡被注册时,v就指向net_device结构,nr_to_call表示准备最多通知几个,-1表示整条链都通知,nr_calls非空的话,返回通知了多少个.
每个被执行的notifier_block回调函数的返回值可能取值为以下几个:
- NOTIFY_DONE:表示对相关的事件类型不关心;
- NOTIFY_OK:顺利执行;
- NOTIFY_BAD:执行有错;
- NOTIFY_STOP:停止执行后面的回调函数;
- NOTIFY_STOP_MASK:停止执行的掩码;
- Notifier_call_chain()把最后一个被调用的回调函数的返回值作为它的返回值.
4. 应用实例
在这里,写了一个简单的通知链表的代码.实际上,整个通知链的编写也就两个过程:
- 首先是定义自己的通知链的头节点,并将要执行的函数注册到自己的通知链中;
- 其次则是由另外的子系统来通知这个链,让其上面注册的函数运行.
这里将第一个过程分成了两步来写,第一步是定义了头节点和一些自定义的注册函数(针对该头节点的),第二步则是使用自定义的注册函数注册了一些通知链节点.分别在代码buildchain.c与regchain.c中.发送通知信息的代码为notify.c.
清单9. buildchain.c
它的作用是自定义一个通知链表test_chain,然后再自定义两个函数分别向这个通知链中加入或删除节点,最后再定义一个函数通知这个test_chain链
buildchain.c清单10. regchain.c
该代码的作用是将test_notifier1,test_notifier2,test_notifier3这三个节点加到之前定义的test_chain这个通知链表上,同时每个节点都注册了一个函数
regchain.c清单11. notify.c
该代码的作用就是向test_chain通知链中发送消息,让链中的函数运行
notify.c清单12. Makefile
1 obj-m:=buildchain.o regchain.o notify.o 2 CURRENT_PATH := $(shell pwd) 3 LINUX_KERNEL := $(shell uname -r) 4 KERNELDIR := /usr/src/linux-headers-$(LINUX_KERNEL) 5 6 all: 7 make -C $(KERNELDIR) M=$(CURRENT_PATH) modules 8 9 clean: 10 11 make -C $(KERNELDIR) M=$(CURRENT_PATH) clean
清单13. compile&load
1 #make 2 3 #insmod buildchain.ko 4 #insmod regchain.ko 5 #insmod notify.ko
清单14. result
1 init_notifier 2 Begin to register: 3 register test_notifier1 completed 4 register test_notifier2 completed 5 register test_notifier3 completed 6 Begin to notify: 7 ============================== 8 In Event 1: Event Number is 1 9 In Event 2: Event Number is 1 10 In Event 3: Event Number is 1 11 ==============================
5. 通知头结构的有关宏
清单15. Head-macro
1 /*以下是一些宏来初始化各种类型的通知头结构, 一般在程序中使用*/
2 #define ATOMIC_INIT_NOTIFIER_HEAD(name) do{
3 spin_lock_init(&(name)->lock);
4 (name)->head =NULL;
5 } while (0)
6
7 #define BLOCKING_INIT_NOTIFIER_HEAD(name) do{
8 init_rwsem(&(name)->rwsem);
9 (name)->head =NULL;
10 } while (0)
11
12 #define RAW_INIT_NOTIFIER_HEAD(name) do{
13 (name)->head =NULL;
14 } while (0)
15
16
17 /*以下这些宏也是用来初始化各种类型的通知头结构,但是在参数定义时使用（即作为赋值的右半部分，作为等号右边的部分）*/
18 #define ATOMIC_NOTIFIER_INIT(name){
19 .lock =__SPIN_LOCK_UNLOCKED(name.lock),
20 .head = NULL }
21 #define BLOCKING_NOTIFIER_INIT(name){
22 .rwsem =__RWSEM_INITIALIZER((name).rwsem),
23 .head = NULL }
24 #defineRAW_NOTIFIER_INIT(name) {
25 .head = NULL }
26
27 /*注意, 没有定义scru通知头结构的初始化, 因为scru是不能静态初始化的.*/
28
29
30 /*以下这些宏用来直接定义通知头结构*/
31 #defineATOMIC_NOTIFIER_HEAD(name)
32 struct atomic_notifier_head name=
33 ATOMIC_NOTIFIER_INIT(name)
34 #defineBLOCKING_NOTIFIER_HEAD(name)
35 struct blocking_notifier_head name=
36 BLOCKING_NOTIFIER_INIT(name)
37 #defineRAW_NOTIFIER_HEAD(name)
38 struct raw_notifier_head name=
39 RAW_NOTIFIER_INIT(name)
6. 扩展的通知块操作
扩展的通知块操作功能和基本通知块类似,但使用了扩展的结构中的参数保证操作的安全
6.1 原子通知块
6.1.1 登记
1 /**
2 * atomic_notifier_chain_register - Add notifier to an atomic notifier chain
3 * @nh: Pointer to head of the atomic notifier chain
4 * @n: New entry in notifier chain
5 *
6 * Adds a notifier to an atomic notifier chain.
7 *
8 * Currently always returns zero.
9 */
10 /*只在基本通知登记操作前后加锁进行保护*/
11 int atomic_notifier_chain_register(struct atomic_notifier_head *nh,
12 struct notifier_block *n)
13 {
14 unsigned long flags;
15 int ret;
16
17 spin_lock_irqsave(&nh->lock, flags); /*加锁*/
18 ret = notifier_chain_register(&nh->head, n);
19 spin_unlock_irqrestore(&nh->lock, flags); /*解锁*/
20 return ret;
21 }
22 EXPORT_SYMBOL_GPL(atomic_notifier_chain_register);
6.1.2 撤销
1 /**
2 * atomic_notifier_chain_unregister - Remove notifier from an atomic notifier chain
3 * @nh: Pointer to head of the atomic notifier chain
4 * @n: Entry to remove from notifier chain
5 *
6 * Removes a notifier from an atomic notifier chain.
7 *
8 * Returns zero on success or %-ENOENT on failure.
9 */
10 /*只是在基本通知块撤销操作前后加锁解锁进行保护*/
11 int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh,
12 struct notifier_block *n)
13 {
14 unsigned long flags;
15 int ret;
16
17 spin_lock_irqsave(&nh->lock, flags); /*加锁*/
18 ret = notifier_chain_unregister(&nh->head, n);
19 spin_unlock_irqrestore(&nh->lock, flags); /*解锁*/
20 synchronize_rcu(); /*同步rcu, 等待一个grace period*/
21 return ret;
22 }
23 EXPORT_SYMBOL_GPL(atomic_notifier_chain_unregister);
6.1.3 原子回调
1 /**
2 * __atomic_notifier_call_chain - Call functions in an atomic notifier chain
3 * @nh: Pointer to head of the atomic notifier chain
4 * @val: Value passed unmodified to notifier function
5 * @v: Pointer passed unmodified to notifier function
6 * @nr_to_call: See the comment for notifier_call_chain.
7 * @nr_calls: See the comment for notifier_call_chain.
8 *
9 * Calls each function in a notifier chain in turn. The functions
10 * run in an atomic context, so they must not block.
11 * This routine uses RCU to synchronize with changes to the chain.
12 *
13 * If the return value of the notifier can be and'ed
14 * with %NOTIFY_STOP_MASK then atomic_notifier_call_chain()
15 * will return immediately, with the return value of
16 * the notifier function which halted execution.
17 * Otherwise the return value is the return value
18 * of the last notifier function called.
19 */
20 int __kprobes __atomic_notifier_call_chain(struct atomic_notifier_head *nh,
21 unsigned long val, void *v,
22 int nr_to_call, int *nr_calls)
23 {
24 int ret;
25
26 rcu_read_lock(); /*禁止抢占*/
27 ret = notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls); /*使用基本通知块回调*/
28 rcu_read_unlock(); /*使能抢占*/
29 return ret;
30 }
31 EXPORT_SYMBOL_GPL(__atomic_notifier_call_chain);
32
33 /*这个函数是在原子操作上下文中调用, 是不能阻塞的*/
34 int __kprobes atomic_notifier_call_chain(struct atomic_notifier_head *nh,
35 unsigned long val, void *v)
36 {
37 return __atomic_notifier_call_chain(nh, val, v, -1, NULL);
38 }
39 EXPORT_SYMBOL_GPL(atomic_notifier_call_chain);
6.2 可阻塞通知块
6.2.1 登记
1 /**
2 * blocking_notifier_chain_register - Add notifier to a blocking notifier chain
3 * @nh: Pointer to head of the blocking notifier chain
4 * @n: New entry in notifier chain
5 *
6 * Adds a notifier to a blocking notifier chain.
7 * Must be called in process context.
8 *
9 * Currently always returns zero.
10 */
11 int blocking_notifier_chain_register(struct blocking_notifier_head *nh,
12 struct notifier_block *n)
13 {
14 int ret;
15
16 /*
17 * This code gets used during boot-up, when task switching is
18 * not yet working and interrupts must remain disabled. At
19 * such times we must not call down_write().
20 */
21 if (unlikely(system_state == SYSTEM_BOOTING)) /*此时是不能阻塞*/
22 return notifier_chain_register(&nh->head, n);
23
24 down_write(&nh->rwsem); /*使用信号灯进行同步, 可能阻塞*/
25 ret = notifier_chain_register(&nh->head, n); /*基本登记函数*/
26 up_write(&nh->rwsem); /*释放信号量*/
27 return ret;
28 }
29 EXPORT_SYMBOL_GPL(blocking_notifier_chain_register);
6.2.2 撤销
1 /**
2 * blocking_notifier_chain_unregister - Remove notifier from a blocking notifier chain
3 * @nh: Pointer to head of the blocking notifier chain
4 * @n: Entry to remove from notifier chain
5 *
6 * Removes a notifier from a blocking notifier chain.
7 * Must be called from process context.
8 *
9 * Returns zero on success or %-ENOENT on failure.
10 */
11 /*该函数是在进程处理过程中调用,可阻塞*/
12 int blocking_notifier_chain_unregister(struct blocking_notifier_head *nh,
13 struct notifier_block *n)
14 {
15 int ret;
16
17 /*
18 * This code gets used during boot-up, when task switching is
19 * not yet working and interrupts must remain disabled. At
20 * such times we must not call down_write().
21 */
22 if (unlikely(system_state == SYSTEM_BOOTING))
23 return notifier_chain_unregister(&nh->head, n);
24
25 down_write(&nh->rwsem);
26 ret = notifier_chain_unregister(&nh->head, n);
27 up_write(&nh->rwsem);
28 return ret;
29 }
30 EXPORT_SYMBOL_GPL(blocking_notifier_chain_unregister);
6.2.3 回调
1 /**
2 * __blocking_notifier_call_chain - Call functions in a blocking notifier chain
3 * @nh: Pointer to head of the blocking notifier chain
4 * @val: Value passed unmodified to notifier function
5 * @v: Pointer passed unmodified to notifier function
6 * @nr_to_call: See comment for notifier_call_chain.
7 * @nr_calls: See comment for notifier_call_chain.
8 *
9 * Calls each function in a notifier chain in turn. The functions
10 * run in a process context, so they are allowed to block.
11 *
12 * If the return value of the notifier can be and'ed
13 * with %NOTIFY_STOP_MASK then blocking_notifier_call_chain()
14 * will return immediately, with the return value of
15 * the notifier function which halted execution.
16 * Otherwise the return value is the return value
17 * of the last notifier function called.
18 */
19 int __blocking_notifier_call_chain(struct blocking_notifier_head *nh,
20 unsigned long val, void *v,
21 int nr_to_call, int *nr_calls)
22 {
23 int ret = NOTIFY_DONE;
24
25 /*
26 * We check the head outside the lock, but if this access is
27 * racy then it does not matter what the result of the test
28 * is, we re-check the list after having taken the lock anyway:
29 */
30 if (rcu_dereference(nh->head)) {
31 down_read(&nh->rwsem);
32 ret = notifier_call_chain(&nh->head, val, v, nr_to_call,
33 nr_calls);
34 up_read(&nh->rwsem);
35 }
36 return ret;
37 }
38 EXPORT_SYMBOL_GPL(__blocking_notifier_call_chain);
39 /*在进行上下文中调用, 可以阻塞*/
40 int blocking_notifier_call_chain(struct blocking_notifier_head *nh,
41 unsigned long val, void *v)
42 {
43 return __blocking_notifier_call_chain(nh, val, v, -1, NULL);
44 }
45 EXPORT_SYMBOL_GPL(blocking_notifier_call_chain);
6.3 原始通知块操作
1 /*
2 * Raw notifier chain routines. There is no protection;
3 * the caller must provide it. Use at your own risk!
4 */
5
6 /**
7 * raw_notifier_chain_register - Add notifier to a raw notifier chain
8 * @nh: Pointer to head of the raw notifier chain
9 * @n: New entry in notifier chain
10 *
11 * Adds a notifier to a raw notifier chain.
12 * All locking must be provided by the caller.
13 *
14 * Currently always returns zero.
15 */
16 /*
17 和基本原始块操作完全相同*/
18 int raw_notifier_chain_register(struct raw_notifier_head *nh,
19 struct notifier_block *n)
20 {
21 return notifier_chain_register(&nh->head, n);
22 }
23 EXPORT_SYMBOL_GPL(raw_notifier_chain_register);
24
25 /**
26 * raw_notifier_chain_unregister - Remove notifier from a raw notifier chain
27 * @nh: Pointer to head of the raw notifier chain
28 * @n: Entry to remove from notifier chain
29 *
30 * Removes a notifier from a raw notifier chain.
31 * All locking must be provided by the caller.
32 *
33 * Returns zero on success or %-ENOENT on failure.
34 */
35 int raw_notifier_chain_unregister(struct raw_notifier_head *nh,
36 struct notifier_block *n)
37 {
38 return notifier_chain_unregister(&nh->head, n);
39 }
40 EXPORT_SYMBOL_GPL(raw_notifier_chain_unregister);
41
42 /**
43 * __raw_notifier_call_chain - Call functions in a raw notifier chain
44 * @nh: Pointer to head of the raw notifier chain
45 * @val: Value passed unmodified to notifier function
46 * @v: Pointer passed unmodified to notifier function
47 * @nr_to_call: See comment for notifier_call_chain.
48 * @nr_calls: See comment for notifier_call_chain
49 *
50 * Calls each function in a notifier chain in turn. The functions
51 * run in an undefined context.
52 * All locking must be provided by the caller.
53 *
54 * If the return value of the notifier can be and'ed
55 * with %NOTIFY_STOP_MASK then raw_notifier_call_chain()
56 * will return immediately, with the return value of
57 * the notifier function which halted execution.
58 * Otherwise the return value is the return value
59 * of the last notifier function called.
60 */
61 int __raw_notifier_call_chain(struct raw_notifier_head *nh,
62 unsigned long val, void *v,
63 int nr_to_call, int *nr_calls)
64 {
65 return notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls);
66 }
67 EXPORT_SYMBOL_GPL(__raw_notifier_call_chain);
68
69 int raw_notifier_call_chain(struct raw_notifier_head *nh,
70 unsigned long val, void *v)
71 {
72 return __raw_notifier_call_chain(nh, val, v, -1, NULL);
73 }
74 EXPORT_SYMBOL_GPL(raw_notifier_call_chain);
6.4 SRCU通知块
6.4.1 登记
1 /**
2 * srcu_notifier_chain_register - Add notifier to an SRCU notifier chain
3 * @nh: Pointer to head of the SRCU notifier chain
4 * @n: New entry in notifier chain
5 *
6 * Adds a notifier to an SRCU notifier chain.
7 * Must be called in process context.
8 *
9 * Currently always returns zero.
10 */
11 /*必须在进程的上下文中调用, 和blocking通知类似*/
12 int srcu_notifier_chain_register(struct srcu_notifier_head *nh,
13 struct notifier_block *n)
14 {
15 int ret;
16
17 /*
18 * This code gets used during boot-up, when task switching is
19 * not yet working and interrupts must remain disabled. At
20 * such times we must not call mutex_lock().
21 */
22 if (unlikely(system_state == SYSTEM_BOOTING))
23 return notifier_chain_register(&nh->head, n);
24
25 mutex_lock(&nh->mutex);
26 ret = notifier_chain_register(&nh->head, n);
27 mutex_unlock(&nh->mutex);
28 return ret;
29 }
30 EXPORT_SYMBOL_GPL(srcu_notifier_chain_register);
6.4.2 撤销
1 /**
2 * srcu_notifier_chain_unregister - Remove notifier from an SRCU notifier chain
3 * @nh: Pointer to head of the SRCU notifier chain
4 * @n: Entry to remove from notifier chain
5 *
6 * Removes a notifier from an SRCU notifier chain.
7 * Must be called from process context.
8 *
9 * Returns zero on success or %-ENOENT on failure.
10 */
11 /*必须在进程的上下文中调用, 和blocking通知类似*/
12 int srcu_notifier_chain_unregister(struct srcu_notifier_head *nh,
13 struct notifier_block *n)
14 {
15 int ret;
16
17 /*
18 * This code gets used during boot-up, when task switching is
19 * not yet working and interrupts must remain disabled. At
20 * such times we must not call mutex_lock().
21 */
22 if (unlikely(system_state == SYSTEM_BOOTING))
23 return notifier_chain_unregister(&nh->head, n);
24
25 mutex_lock(&nh->mutex);
26 ret = notifier_chain_unregister(&nh->head, n);
27 mutex_unlock(&nh->mutex);
28 synchronize_srcu(&nh->srcu);
29 return ret;
30 }
31 EXPORT_SYMBOL_GPL(srcu_notifier_chain_unregister);
6.4.3 回调
1 /**
2 * __srcu_notifier_call_chain - Call functions in an SRCU notifier chain
3 * @nh: Pointer to head of the SRCU notifier chain
4 * @val: Value passed unmodified to notifier function
5 * @v: Pointer passed unmodified to notifier function
6 * @nr_to_call: See comment for notifier_call_chain.
7 * @nr_calls: See comment for notifier_call_chain
8 *
9 * Calls each function in a notifier chain in turn. The functions
10 * run in a process context, so they are allowed to block.
11 *
12 * If the return value of the notifier can be and'ed
13 * with %NOTIFY_STOP_MASK then srcu_notifier_call_chain()
14 * will return immediately, with the return value of
15 * the notifier function which halted execution.
16 * Otherwise the return value is the return value
17 * of the last notifier function called.
18 */
19 int __srcu_notifier_call_chain(struct srcu_notifier_head *nh,
20 unsigned long val, void *v,
21 int nr_to_call, int *nr_calls)
22 {
23 int ret;
24 int idx;
25
26 idx = srcu_read_lock(&nh->srcu); /*使用srcu来加锁*/
27 ret = notifier_call_chain(&nh->head, val, v, nr_to_call, nr_calls);
28 srcu_read_unlock(&nh->srcu, idx);
29 return ret;
30 }
31 EXPORT_SYMBOL_GPL(__srcu_notifier_call_chain);
32 /*在进程的上下文中调用, 可以阻塞*/
33 int srcu_notifier_call_chain(struct srcu_notifier_head *nh,
34 unsigned long val, void *v)
35 {
36 return __srcu_notifier_call_chain(nh, val, v, -1, NULL);
37 }
38 EXPORT_SYMBOL_GPL(srcu_notifier_call_chain);
6.4.4 初始化
1 /**
2 * srcu_init_notifier_head - Initialize an SRCU notifier head
3 * @nh: Pointer to head of the srcu notifier chain
4 *
5 * Unlike other sorts of notifier heads, SRCU notifier heads require
6 * dynamic initialization. Be sure to call this routine before
7 * calling any of the other SRCU notifier routines for this head.
8 *
9 * If an SRCU notifier head is deallocated, it must first be cleaned
10 * up by calling srcu_cleanup_notifier_head(). Otherwise the head's
11 * per-cpu data (used by the SRCU mechanism) will leak.
12 */
13 /*因为SRCU通知不能通过宏来初始化,必须要专门定义一个初始化函数来初始化srcu的通知块参数*/
14 void srcu_init_notifier_head(struct srcu_notifier_head *nh)
15 {
16 mutex_init(&nh->mutex);
17 if (init_srcu_struct(&nh->srcu) < 0)
18 BUG();
19 nh->head = NULL;
20 }
21 EXPORT_SYMBOL_GPL(srcu_init_notifier_head);
本文参考：
http://blog.csdn.net/tommy_wxie/article/details/7963926
http://blog.sina.com.cn/s/blog_5426448c0100ntqb.html
http://hi.baidu.com/yskcg/item/8947658d7cbdd8c0b07154a5
http://geyingzhen.blog.163.com/blog/static/6558665520091165531675/