Mutex/Semphore/Binary Semphore的异同

 

Mutex是一把钥匙，一个人拿了就可进入一个房间，出来的时候把钥匙交给队列的第一个。一般的用法是用于串行化对critical section代码的访问，保证这段代码不会被并行的运行。

 

Semaphore是一件可以容纳N人的房间，如果人不满就可以进去，如果人满了，就要等待有人出来。对于N=1的情况，称为binary semaphore。一般的用法是，用于限制对于某一资源的同时访问。

 

Binary semaphore与Mutex的差异：

在有的系统中Binary semaphore与Mutex是没有差异的。在有的系统上，主要的差异是mutex一定要由获得锁的进程来释放。而semaphore可以由其它进程释放（这时的semaphore实际就是个原子的变量，大家可以加或减），因此semaphore可以用于进程间同步。Semaphore的同步功能是所有系统都支持的，而Mutex能否由其他进程释放则未定，因此建议mutex只用于保护critical section。而semaphore则用于保护某变量，或者同步。

 

另一个概念是spin lock，这是一个内核态概念。spin lock与semaphore的主要区别是spin lock是busy waiting，而semaphore是sleep。对于可以sleep的进程来说，busy waiting当然没有意义。对于单CPU的系统，busy waiting当然更没意义（没有CPU可以释放锁）。因此，只有多CPU的内核态非进程空间，才会用到spin lock。Linux kernel的spin lock在非SMP的情况下，只是关irq，没有别的操作，用于确保该段程序的运行不会被打断。其实也就是类似mutex的作用，串行化对critical section的访问。但是mutex不能保护中断的打断，也不能在中断处理程序中被调用。而spin lock也一般没有必要用于可以sleep的进程空间。

 

上述摘自：http://www.yuanma.org/data/2008/0612/article_3076.htm

 

关于semaphore和mutex的区别，网上有著名的厕所理论（http://koti.mbnet.fi/niclasw/MutexSemaphore.html）：

Mutex:Is a key to a toilet. One person can have the key - occupy the toilet - at the time. When finished, the person gives (frees) the key to the next person in the queue.Officially: “Mutexes are typically used to serialise access to a section of re-entrant code that cannot be executed concurrently by more than one thread. A mutex object only allows one thread into a controlled section, forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section.”
Ref: Symbian Developer Library(A mutex is really a semaphore with value 1.)

Semaphore:

Is the number of free identical toilet keys. Example, say we have four toilets with identical locks and keys. The semaphore count - the count of keys - is set to 4 at beginning (all four toilets are free), then the count value is decremented as people are coming in. If all toilets are full, ie. there are no free keys left, the semaphore count is 0. Now, when eq. one person leaves the toilet, semaphore is increased to 1 (one free key), and given to the next person in the queue.

Officially: “A semaphore restricts the number of simultaneous users of a shared resource up to a maximum number. Threads can request access to the resource (decrementing the semaphore), and can signal that they have finished using the resource (incrementing the semaphore).”
Ref: Symbian Developer Library

所以，mutex就是一个binary semaphore （值就是0或者1）。但是他们的区别又在哪里呢？主要有两个方面：

    * 初始状态不一样：mutex的初始值是1（表示锁available），而semaphore的初始值是0（表示unsignaled的状态）。随后的操 作基本一样。mutex_lock和sem_post都把值从0变成1，mutex_unlock和sem_wait都把值从1变成0（如果值是零就等 待）。初始值决定了：虽然mutex_lock和sem_wait都是执行V操作，但是sem_wait将立刻将当前线程block住，直到有其他线程 post；mutex_lock在初始状态下是可以进入的。
    * 用法不一样（对称 vs. 非对称）：这里说的是“用法”。Semaphore实现了signal，但是mutex也有signal（当一个线程lock后另外一个线程 unlock，lock住的线程将收到这个signal继续运行）。在mutex的使用中，模型是对称的。unlock的线程也要先lock。而 semaphore则是非对称的模型，对于一个semaphore，只有一方post，另外一方只wait。就拿上面的厕所理论来说，mutex是一个钥 匙不断重复的使用，传递在各个线程之间，而semaphore择是一方不断的制造钥匙，而供另外一方使用（另外一方不用归还）。

前面的实验证明，mutex确实能够做到post和wait的功能，只是大家不用而已，因为它是“mutex”不是semaphore。

上述摘自：http://hi.baidu.com/d_life/blog/item/c52d7501a96629de267fb54c.html

 

Mutex和Semphore的官方解释：

 

Mutex:

 

Is a key to a toilet. One person can have the key - occupy the toilet - at the time. When finished, the person gives (frees) the key to the next person in the queue.

 

Officially: "Mutexes are typically used to serialise access to a section of re-entrant code that cannot be executed concurrently by more than one thread. A mutex object only allows one thread into a controlled section, forcing other threads which attempt to gain access to that section to wait until the first thread has exited from that section." Ref: Symbian Developer Library(A mutex is really a semaphore with value 1.)

 

Semaphore:

 

Is the number of free identical toilet keys. Example, say we have four toilets with identical locks and keys. The semaphore count - the count of keys - is set to 4 at beginning (all four toilets are free), then the count value is decremented as people are coming in. If all toilets are full, ie. there are no free keys left, the semaphore count is 0. Now, when eq. one person leaves the toilet, semaphore is increased to 1 (one free key), and given to the next person in the queue.

 

Officially: "A semaphore restricts the number of simultaneous users of a shared resource up to a maximum number. Threads can request access to the resource (decrementing the semaphore), and can signal that they have finished using the resource (incrementing the semaphore)." Ref: Symbian Developer Library