$(1)$ Please explain the three synchronization mechanisms for threads to access a shared data item in UNIX systems. $(6\%)$

$(2)$ Suppose that three processes P$1,$ P$2,$ and P$3$ respectively have timestamps $17,25,$ and $30,$ and P$2$ currently holds a resource requested by both P$1$ and P$3.$ Please give the results of wait$-$die and wound$-$wait schemes used to handle deadlocks. $(4\%)$

$(3)$ Please explain how the two$-$phase locking protocol works to check the correctness of a nonserial schedule for atomic transactions. $(5\%)$