OS02经典进程同步问题2014-2015-2.ppt

* * * * 同时wait操作 AND同步操作 * 只要有一个读者，就不允许写了。 * 为实现Reader与Writer进程间在读或写时的互斥而设置了一个互斥信号量Wmutex。另外，再设置一个整型变量Readcount表示正在读的进程数目。由于只要有一个Reader进程在读，便不允许Writer进程去写。因此，仅当Readcount=0，表示尚无Reader进程在读时，Reader进程才需要执行Wait(Wmutex)操作。若Wait(Wmutex)操作成功，Reader进程便可去读，相应地，做Readcount+1操作。同理，仅当Reader进程在执行了Readcount减1操作后其值为0时，才须执行signal(Wmutex)操作，以便让Writer进程写。又因为Readcount是一个可被多个Reader进程访问的临界资源，因此，也应该为它设置一个互斥信号量rmutex。 问题分析： 写者来说：主要写，lock 读者来说：只要有一个，意味不允许再写，加锁。再来，不必，最后通知 　　互斥锁：读文件时不能写，写文件时不能读文件 　　读进程：允许多个文件读，读进程时0时，锁定文件，读文件进程1时，解锁；读进程数0时，说明读进程拥有锁 　　写进程:拥有锁时写文件 * * * 　　　 * 　　　 * * * * * * 顾客：顾客到达 理发师：理发师准备好 * 理发师：关心的条件：有无顾客来理发 顾客： 理发师状态：sleep 等待 理发 * P(barbers):请求理发师资源，禁止他再为其他用户服务 . he tries to get exclusive access of the barber to cut his hair. * When the barber first comes to the shop, he looks out for any customers i.e. calls P(Customers), if there are none he goes to sleep. Now when a customer arrives, the customer tries to get access to the accessSeats mutex i.e. he calls P(accessSeats), thereby setting a lock. If no free seat (barber chair and waiting chairs) is available he releases the lock i.e. does a V(accessSeats) and leaves the shop. If there is a free seat he first decreases the number of free seats by one and he calls V(Customers) to notify the barber that he wants to cut. Then the customer releases the lock on the accessSeats mutex by calling V(accessSeats). Meanwhile when V(Customers) was called the barber awakes. The barber locks the accessSeats mutex as he wants to increase the number of free seats available, as the just arrived customer may sit on the barber’s chair if that is free. Now the barber releases the lock on the accessSeats mutex so that other customers can access it to the see the status of the free seats. The barber now calls a V(Barber), i.e. he tells the customer that he is available to cut. The customer now calls a P(Barber), i.e. he tries to get exclusive access of the barber to cut his hair. The customer gets a haircut from the barber and as soon a