Mutex and Semaphore

concep ref

implementation ref1

so mutex is more efficient, in other words, it takes less overhead!

and there's sth about semaphore ref

semaphores
    A semaphore is a special counter.  It has an initial value and two
    operations, P and V, for changing that value.  (P and V are
    initials for words in Dutch; think of them as Pause and something
    else [Voice maybe?]).  A semaphore keeps track of the difference
    between the number of P and V operations that have occurred.  A P
    operation is delayed (the process is de-scheduled) until #P-#V <=
    C, the initial value of the semaphore.