互斥锁LOCK()与RLOCK()

Posted 2021-03-02 zuzhuangmengxiang

资源总是有限的，程序运行如果对同一个对象进行操作，则有可能造成资源竞争，也可能导致读写混乱，此时需要引入锁。

锁提供如下方法： 
1.Lock.acquire([blocking])    # 上锁
2.Lock.release()      # 解锁
3.threading.Lock()      # 加载线程的锁对象，是一个基本的锁对象，一次只能一个锁定，其余锁请求，需等待锁释放后才能获取

4.threading.RLock() 多重锁，在同一线程中可用被多次acquire。如果使用RLock，那么acquire和release必须成对出现， 
调用了n次acquire锁请求，则必须调用n次的release才能在线程中释放锁对象

例如： 
无锁：

 1 import threading
 2 import time
 3 
 4 g_num = 0
 5 
 6 
 7 def work1(num):
 8     for i in range(num):
 9         global g_num
10         g_num += 1
11     print("___in work1,g_num is %d" % g_num)
12 
13 
14 def work2(num):
15     for i in range(num):
16         global g_num
17         g_num += 1
18     print("___in work2,g_num is %d" % g_num)
19 
20 
21 t1 = threading.Thread(target=work1, args=(1000000,))
22 t2 = threading.Thread(target=work2, args=(1000000,))
23 t1.start()
24 t2.start()
25 
26 time.sleep(3)
27 
28 print("___g_num is %d" % g_num)

　　运行结果：

1 ___in work1,g_num is 1194618
2 ___in work2,g_num is 1334899
3 ___g_num is 1334899

引入锁：

 1 import threading
 2 import time
 3 
 4 # 定义一个全局变量
 5 g_num = 0
 6 
 7 
 8 def work1(num):
 9     global g_num
10     for i in range(num):
11         # 互斥锁上锁和解锁间包含的代码越少越好
12         mutex.acquire()
13         g_num += 1
14         mutex.release()
15     print("___in work1,g_num is %d" % g_num)
16 
17 
18 def work2(num):
19     global g_num
20     for i in range(num):
21         mutex.acquire()
22         g_num += 1
23         mutex.release()
24     print("___in work2,g_num is %d" % g_num)
25 
26 
27 # 创建一个互斥锁，默认是没有上锁的
28 mutex = threading.Lock()
29 
30 
31 t1 = threading.Thread(target=work1, args=(1000000,))
32 t2 = threading.Thread(target=work2, args=(1000000,))
33 t1.start()
34 t2.start()
35 
36 time.sleep(3)
37 
38 print("___g_num is %d" % g_num)

　　运行结果：

1 ___in work1,g_num is 1945701
2 ___in work2,g_num is 2000000
3 ___g_num is 2000000

其中： 
　　lock=threading.Lock()加载锁的方法也可以换成lock=threading.RLock()

　　如果将上面的work1改为：

1 mutex.acquire()
2 global g_num
3 mutexk.acquire()
4     for i in range(num):
5         g_num += 1
6         mutex.release()
7         print("___in work1,g_num is %d" % g_num)
8         mutexk.release()

　　那么： 
　　lock=threading.Lock() 加载的锁，则一直处于等待中，锁等待 
　　而lock=threading.RLock() 运行正常

lock()避免死锁的解决方法：

　　1.程序设置时要尽量避免（银行家算法）

　　2.添加超时时间