读者优先问题。操作限制：1）写-写互斥；2）读-写互斥；3）读-读允许； 读者优先的附加限制：如果一个读者申请进行读操作时已有另一读者正在进行读操作，则该读者可直接开始读操作。-readers priority. Operating restrictions : a) write-write contradict; 2) read-write mutually exclusive; 3) Reading-Reading allowed; Readers priority to the addit