The readers-writers problem has several variations,all involving priorities.
1、The simplest one is called the first readers-writers problem,requires that
no readers will be kept waiting unless a writer has already obtained permission
to use the shared object.
2、The second readers-writers problem requires that,once a writer is ready,that
writer performs its write as soon as possible.In other words,if a waiter is waiting
to access the object,no new readers may start reading.
A sol