看过Linux内核的同学都知道，Linux内核中除了有semaphore之外，还有一个mutex lock。前者我们的操作系统教科书称之为信号量，后者不知道教科书有没有具体的名称，但是在Linux内核中，它的称谓是"互斥锁"或者“互斥体”(总 之，称谓不是问题)。为了提升一下本帖的理论密度，特从Wiki中摘录一段关于semaphore的描述：
“In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment.
A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record as units are required or become free, and if necessary wait until a unit of the resource becomes available. Semaphores are a useful tool in the prevention of race conditions and deadlocks; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores (same functionality that mutexes have).”。
其中关键信息主要是“a semaphore is a data type for controlling access by multiple processes to a common resource in a parallel programming environment... Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores (same functionality that mutexes have)”，也即信号量在并行处理环境下对多个processes访问某个公共资源进行保护，后面提到的binary semaphore，本质上应该就是mutex了，从same functionality that mutexes have这句话来看，mutex和binary semaphore功能应该相同。从以上的文字中显然可以看到，相对mutex而言信号量的适用范围更广(mutex只是信号量的用途之一)，这个我们接 下来在后续的Linux源码中也可以看到这其中某些细微之处的区分。
*注：昨天写这个帖子时手头没有操作系统方面的书籍拿来参考，今天我翻了一下《现代操作系统》(陈向群等译，机械工业出版社 1999年11月第1 版)， 关于这个话题，书里明确提到的只有"2.2.5 信号量"，至于mutex，书中并没有作为一个独立的概念提出来，只是在讲信号量时提到了上面所说的binary semaphore，并且说“信号量mutex(应该是指binary semaphore)用于互斥...互斥是避免混乱所必需的操作...信号量的另一种用途是用于实现同步(synchronization)。信号量 full和empty用来保证一定的事件顺序发生或不发生。在本例中，它们保证缓冲区满的时候生产者停止运行，或者当缓冲区空的时候消费者停止运行。这种 用法与互斥是不同的” --- P30-31 *
OK，理论上的概念有了，那么就来看看实际当中Linux下的semaphone到底长的啥样。以下是semaphore在Linux源码中的定义，源码来自3.2.9：
如果count=1的话，那么semaphore就可以用来进行互斥操作了，早先内核源码中曾有一个专门的宏用来定义一个count=1的信号量DECLARE_MUTEX:
因为我们知道在Linux内核源码中还有一个DEFINE_MUTEX宏，所以Marcin Slusarz同学认为DECLARE_MUTEX宏容易让人困惑，毕竟它其实只是定义了一个count=1的信号量，因此该同学早在08年就向内核社区 提交了一个PATCH，要求Rename DECLARE_MUTEX to DEFINE_SEMAPHORE(https://lkml.org/lkml/2008/10/26/74)， 这个PATCH最终被社区所接受(应该是在2.6.35和2.6.39版本之间，2.6.39已经没有了DECLARE_MUTEX，取而代之的是 DEFINE_SEMAPHORE，但2.6.35还有，我当时在写《深入Linux设备驱动程序内核机制》时，最早引用的是2.6.35的版本，虽然在 写作的中晚期将内核版本切换到2.6.39并在定稿阶段曾试图将之前写的文档全部修正到39版，但是DECLARE_MUTEX的残留显然是条漏网之 鱼...)。因为只是rename，所以DEFINE_SEMAPHORE的定义和原来的DECLARE_MUTEX完全相同。
那么既然count=1的信号量可以用来完成mutex lock的功能，那么内核何必再多此一举弄出个mutex lock出来呢？
“In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment.
A useful way to think of a semaphore is as a record of how many units of a particular resource are available, coupled with operations to safely (i.e., without race conditions) adjust that record as units are required or become free, and if necessary wait until a unit of the resource becomes available. Semaphores are a useful tool in the prevention of race conditions and deadlocks; however, their use is by no means a guarantee that a program is free from these problems. Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores (same functionality that mutexes have).”。
其中关键信息主要是“a semaphore is a data type for controlling access by multiple processes to a common resource in a parallel programming environment... Semaphores which allow an arbitrary resource count are called counting semaphores, while semaphores which are restricted to the values 0 and 1 (or locked/unlocked, unavailable/available) are called binary semaphores (same functionality that mutexes have)”，也即信号量在并行处理环境下对多个processes访问某个公共资源进行保护，后面提到的binary semaphore，本质上应该就是mutex了，从same functionality that mutexes have这句话来看，mutex和binary semaphore功能应该相同。从以上的文字中显然可以看到，相对mutex而言信号量的适用范围更广(mutex只是信号量的用途之一)，这个我们接 下来在后续的Linux源码中也可以看到这其中某些细微之处的区分。
*注：昨天写这个帖子时手头没有操作系统方面的书籍拿来参考，今天我翻了一下《现代操作系统》(陈向群等译，机械工业出版社 1999年11月第1 版)， 关于这个话题，书里明确提到的只有"2.2.5 信号量"，至于mutex，书中并没有作为一个独立的概念提出来，只是在讲信号量时提到了上面所说的binary semaphore，并且说“信号量mutex(应该是指binary semaphore)用于互斥...互斥是避免混乱所必需的操作...信号量的另一种用途是用于实现同步(synchronization)。信号量 full和empty用来保证一定的事件顺序发生或不发生。在本例中，它们保证缓冲区满的时候生产者停止运行，或者当缓冲区空的时候消费者停止运行。这种 用法与互斥是不同的” --- P30-31 *
OK，理论上的概念有了，那么就来看看实际当中Linux下的semaphone到底长的啥样。以下是semaphore在Linux源码中的定义，源码来自3.2.9：
- /* Please don't access any members of this structure directly */
- struct semaphore {
- raw_spinlock_t lock;
- unsigned int count;
- struct list_head wait_list;
- };
- #define DECLARE_MUTEX(name) \
- struct semaphore name = __SEMAPHORE_INITIALIZER(name, 1)
那么既然count=1的信号量可以用来完成mutex lock的功能，那么内核何必再多此一举弄出个mutex lock出来呢？
