一、同步鎖

1.1 多個線程搶占資源的情況

          
            from threading import Thread
import os,time
def work():
    global n
    temp=n
    time.sleep(0.1)
    n=temp-1
if __name__ == '__main__':
    n=100
    l=[]
    for i in range(100):
        p=Thread(target=work)
        l.append(p)
        p.start()
    for p in l:
        p.join()

    print(n) #結果可能為99
          
        

1.1.1 對公共數據的操作

          
            import threading
R=threading.Lock()
R.acquire()
'''
對公共數據的操作
'''
R.release()
          
        

1.2 同步鎖的引用

          
            from threading import Thread,Lock
import os,time
def work():
    global n
    lock.acquire()
    temp=n
    time.sleep(0.1)
    n=temp-1
    lock.release()
if __name__ == '__main__':
    lock=Lock()
    n=100
    l=[]
    for i in range(100):
        p=Thread(target=work)
        l.append(p)
        p.start()
    for p in l:
        p.join()

    print(n) #結果肯定為0，由原來的并發執行變成串行，犧牲了執行效率保證了數據安全
          
        

1.3 互斥鎖與join的區別

          
            #不加鎖:并發執行,速度快,數據不安全
from threading import current_thread,Thread,Lock
import os,time
def task():
    global n
    print('%s is running' %current_thread().getName())
    temp=n
    time.sleep(0.5)
    n=temp-1


if __name__ == '__main__':
    n=100
    lock=Lock()
    threads=[]
    start_time=time.time()
    for i in range(100):
        t=Thread(target=task)
        threads.append(t)
        t.start()
    for t in threads:
        t.join()

    stop_time=time.time()
    print('主:%s n:%s' %(stop_time-start_time,n))

'''
Thread-1 is running
Thread-2 is running
......
Thread-100 is running
主:0.5216062068939209 n:99
'''


#不加鎖:未加鎖部分并發執行,加鎖部分串行執行,速度慢,數據安全
from threading import current_thread,Thread,Lock
import os,time
def task():
    #未加鎖的代碼并發運行
    time.sleep(3)
    print('%s start to run' %current_thread().getName())
    global n
    #加鎖的代碼串行運行
    lock.acquire()
    temp=n
    time.sleep(0.5)
    n=temp-1
    lock.release()

if __name__ == '__main__':
    n=100
    lock=Lock()
    threads=[]
    start_time=time.time()
    for i in range(100):
        t=Thread(target=task)
        threads.append(t)
        t.start()
    for t in threads:
        t.join()
    stop_time=time.time()
    print('主:%s n:%s' %(stop_time-start_time,n))

'''
Thread-1 is running
Thread-2 is running
......
Thread-100 is running
主:53.294203758239746 n:0
'''

# 有的同學可能有疑問:既然加鎖會讓運行變成串行,那么我在start之后立即使用join,就不用加鎖了啊,也是串行的效果啊

# 沒錯:在start之后立刻使用jion,肯定會將100個任務的執行變成串行,毫無疑問,最終n的結果也肯定是0,是安全的,但問題是

# start后立即join:任務內的所有代碼都是串行執行的,而加鎖,只是加鎖的部分即修改共享數據的部分是串行的

# 單從保證數據安全方面,二者都可以實現,但很明顯是加鎖的效率更高.
from threading import current_thread,Thread,Lock
import os,time
def task():
    time.sleep(3)
    print('%s start to run' %current_thread().getName())
    global n
    temp=n
    time.sleep(0.5)
    n=temp-1


if __name__ == '__main__':
    n=100
    lock=Lock()
    start_time=time.time()
    for i in range(100):
        t=Thread(target=task)
        t.start()
        t.join()
    stop_time=time.time()
    print('主:%s n:%s' %(stop_time-start_time,n))

'''
Thread-1 start to run
Thread-2 start to run
......
Thread-100 start to run
主:350.6937336921692 n:0 #耗時是多么的恐怖
'''

）
          
        

二、死鎖與遞歸鎖

進程也有死鎖與遞歸鎖，在進程那里忘記說了，放到這里一起說了。

所謂死鎖：是指兩個或兩個以上的進程或線程在執行過程中， 因爭奪資源而造成的一種互相等待的現象， 若無外力作用，它們都將無法推進下去。此時稱系統處于死鎖狀態或系統產生了死鎖，這些永遠在互相等待的進程稱為死鎖進程，如下就是死鎖

2.1 死鎖

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

解決方法： 遞歸鎖，在Python中為了支持在同一線程中多次請求同一資源，python提供了可重入鎖RLock。

這個RLock內部維護著一個Lock和一個counter變量，counter記錄了acquire的次數，從而使得資源可以被多次require。直到一個線程所有的acquire都被release，其他的線程才能獲得資源。上面的例子如果使用RLock代替Lock，則不會發生死鎖。

2.2 遞歸鎖RLock

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

三、典型問題：科學家吃面

3.1 死鎖問題

          
            import time
from threading import Thread,Lock
noodle_lock = Lock()
fork_lock = Lock()
def eat1(name):
    noodle_lock.acquire()
    print('%s 搶到了面條'%name)
    fork_lock.acquire()
    print('%s 搶到了叉子'%name)
    print('%s 吃面'%name)
    fork_lock.release()
    noodle_lock.release()

def eat2(name):
    fork_lock.acquire()
    print('%s 搶到了叉子' % name)
    time.sleep(1)
    noodle_lock.acquire()
    print('%s 搶到了面條' % name)
    print('%s 吃面' % name)
    noodle_lock.release()
    fork_lock.release()

for name in ['哪吒','nick','tank']:
    t1 = Thread(target=eat1,args=(name,))
    t2 = Thread(target=eat2,args=(name,))
    t1.start()
    t2.start()
          
        

3.2 遞歸鎖解決死鎖問題

          
            import time
from threading import Thread,RLock
fork_lock = noodle_lock = RLock()
def eat1(name):
    noodle_lock.acquire()
    print('%s 搶到了面條'%name)
    fork_lock.acquire()
    print('%s 搶到了叉子'%name)
    print('%s 吃面'%name)
    fork_lock.release()
    noodle_lock.release()

def eat2(name):
    fork_lock.acquire()
    print('%s 搶到了叉子' % name)
    time.sleep(1)
    noodle_lock.acquire()
    print('%s 搶到了面條' % name)
    print('%s 吃面' % name)
    noodle_lock.release()
    fork_lock.release()

for name in ['哪吒','nick','tank']:
    t1 = Thread(target=eat1,args=(name,))
    t2 = Thread(target=eat2,args=(name,))
    t1.start()
    t2.start()