Mutex是一把鑰匙，一個人拿了就可進入一個房間，出來的時候把鑰匙交給隊列的第一個。一般的用法是用於串行化對critical section代碼的訪問，保證這段代碼不會被並行的運行。
Semaphore是一件可以容納N人的房間，如果人不滿就可以進去，如果人滿了，就要等待有人出來。對於N=1的情況，稱爲binary semaphore。一般的用法是，用於限制對於某一資源的同時訪問。
Binary semaphore與Mutex的差異：
在有的系統中Binary semaphore與Mutex是沒有差異的。在有的系統上，主要的差異是mutex一定要由獲得鎖的進程來釋放。而semaphore可以由其它進程釋放（這時的semaphore實際就是個原子的變量，大家可以加或減），因此semaphore可以用於進程間同步。Semaphore的同步功能是所有系統都支持的，而Mutex能否由其他進程釋放則未定，因此建議mutex只用於保護critical section。而semaphore則用於保護某變量，或者同步。
另一個概念是spin lock，這是一個內核態概念。spin lock與semaphore的主要區別是spin lock是busy waiting，而semaphore是sleep。對於可以sleep的進程來說，busy waiting當然沒有意義。對於單CPU的系統，busy waiting當然更沒意義（沒有CPU可以釋放鎖）。因此，只有多CPU的內核態非進程空間，纔會用到spin lock。Linux kernel的spin lock在非SMP的情況下，只是關irq，沒有別的操作，用於確保該段程序的運行不會被打斷。其實也就是類似mutex的作用，串行化對critical section的訪問。但是mutex不能保護中斷的打斷，也不能在中斷處理程序中被調用。而spin lock也一般沒有必要用於可以sleep的進程空間。
上述摘自：http://www.yuanma.org/data/2008/0612/article_3076.htm
關於semaphore和mutex的區別，網上有著名的廁所理論（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就是一個binary semaphore （值就是0或者1）。但是他們的區別又在哪裏呢？主要有兩個方面：
* 初始狀態不一樣：mutex的初始值是1（表示鎖available），而semaphore的初始值是0（表示unsignaled的狀態）。隨後的操 作基本一樣。mutex_lock和sem_post都把值從0變成1，mutex_unlock和sem_wait都把值從1變成0（如果值是零就等 待）。初始值決定了：雖然mutex_lock和sem_wait都是執行V操作，但是sem_wait將立刻將當前線程block住，直到有其他線程 post；mutex_lock在初始狀態下是可以進入的。
* 用法不一樣（對稱 vs. 非對稱）：這裏說的是“用法”。Semaphore實現了signal，但是mutex也有signal（當一個線程lock後另外一個線程 unlock，lock住的線程將收到這個signal繼續運行）。在mutex的使用中，模型是對稱的。unlock的線程也要先lock。而 semaphore則是非對稱的模型，對於一個semaphore，只有一方post，另外一方只wait。就拿上面的廁所理論來說，mutex是一個鑰 匙不斷重複的使用，傳遞在各個線程之間，而semaphore擇是一方不斷的製造鑰匙，而供另外一方使用（另外一方不用歸還）。
前面的實驗證明，mutex確實能夠做到post和wait的功能，只是大家不用而已，因爲它是“mutex”不是semaphore。
上述摘自：http://hi.baidu.com/d_life/blog/item/c52d7501a96629de267fb54c.html
Mutex和Semphore的官方解釋：
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