Difference between Mutex and Semaphore

Difference between Mutex and Semaphore

Source:

先看看网上著名的厕所理论的描述：
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."

(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)."

还有下面的一些理解：
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。