死锁与递归锁

进程也有死锁与递归锁，在进程那里忘记说了，放到这里一切说了额

所谓死锁：是指两个或两个以上的进程或线程在执行过程中，因争夺资源而造成的一种互相等待的现象，若无外力作用，它们都将无法推进下去。此时称系统处于死锁状态或系统产生了死锁，这些永远在互相等待的进程称为死锁进程，如下就是死锁

 1 from threading import Lock as Lock
 2 import time
 3 mutexA=Lock()
 4 mutexA.acquire()
 5 mutexA.acquire()
 6 print(123)
 7 mutexA.release()
 8 mutexA.release()
 9 
10 死锁

解决方法，递归锁，在Python中为了支持在同一线程中多次请求同一资源，python提供了可重入锁RLock。

这个RLock内部维护着一个Lock和一个counter变量，counter记录了acquire的次数，从而使得资源可以被多次require。直到一个线程所有的acquire都被release，其他的线程才能获得资源。上面的例子如果使用RLock代替Lock，则不会发生死锁：

from threading import RLock as Lock
import time
mutexA=Lock()
mutexA.acquire()
mutexA.acquire()
print(123)
mutexA.release()
mutexA.release()

递归锁RLock

典型问题: 科学家吃面

 1 import time
 2 from threading import Thread,Lock
 3 noodle_lock = Lock()
 4 fork_lock = Lock()
 5 def eat1(name):
 6     noodle_lock.acquire()
 7     print('%s 抢到了面条'%name)
 8     fork_lock.acquire()
 9     print('%s 抢到了叉子'%name)
10     print('%s 吃面'%name)
11     fork_lock.release()
12     noodle_lock.release()
13 
14 def eat2(name):
15     fork_lock.acquire()
16     print('%s 抢到了叉子' % name)
17     time.sleep(1)
18     noodle_lock.acquire()
19     print('%s 抢到了面条' % name)
20     print('%s 吃面' % name)
21     noodle_lock.release()
22     fork_lock.release()
23 
24 for name in ['哪吒','egon','yuan']:
25     t1 = Thread(target=eat1,args=(name,))
26     t2 = Thread(target=eat2,args=(name,))
27     t1.start()
28     t2.start()
29 
30 死锁问题

 1 import time
 2 from threading import Thread,RLock
 3 fork_lock = noodle_lock = RLock()
 4 def eat1(name):
 5     noodle_lock.acquire()
 6     print('%s 抢到了面条'%name)
 7     fork_lock.acquire()
 8     print('%s 抢到了叉子'%name)
 9     print('%s 吃面'%name)
10     fork_lock.release()
11     noodle_lock.release()
12 
13 def eat2(name):
14     fork_lock.acquire()
15     print('%s 抢到了叉子' % name)
16     time.sleep(1)
17     noodle_lock.acquire()
18     print('%s 抢到了面条' % name)
19     print('%s 吃面' % name)
20     noodle_lock.release()
21     fork_lock.release()
22 
23 for name in ['哪吒','egon','yuan']:
24     t1 = Thread(target=eat1,args=(name,))
25     t2 = Thread(target=eat2,args=(name,))
26     t1.start()
27     t2.start()
28 
29 递归锁解决死锁问题

死锁 与 递归锁

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