Python中死鎖的形成示例及死鎖情況的防止

#coding=utf-8 
import time 
import threading 
class Account: 
 def __init__(self, _id, balance, lock): 
 self.id = _id 
 self.balance = balance 
 self.lock = lock 
 
 def withdraw(self, amount): 
 self.balance -= amount 
 
 def deposit(self, amount): 
 self.balance += amount 
 
 
def transfer(_from, to, amount): 
 if _from.lock.acquire():#鎖住自己的賬戶 
 _from.withdraw(amount) 
 time.sleep(1)#讓交易時間變長，2個交易線程時間上重疊，有足夠時間來產生死鎖 
 print 'wait for lock...' 
 if to.lock.acquire():#鎖住對方的賬戶 
 to.deposit(amount) 
 to.lock.release() 
 _from.lock.release() 
 print 'finish...' 
 
a = Account('a',1000, threading.Lock()) 
b = Account('b',1000, threading.Lock()) 
threading.Thread(target = transfer, args = (a, b, 100)).start() 
threading.Thread(target = transfer, args = (b, a, 200)).start() 

import threading
from contextlib import contextmanager

# Thread-local state to stored information on locks already acquired
_local = threading.local()

@contextmanager
def acquire(*locks):
 # Sort locks by object identifier
 locks = sorted(locks, key=lambda x: id(x))

 # Make sure lock order of previously acquired locks is not violated
 acquired = getattr(_local,'acquired',[])
 if acquired and max(id(lock) for lock in acquired) >= id(locks[0]):
 raise RuntimeError('Lock Order Violation')

 # Acquire all of the locks
 acquired.extend(locks)
 _local.acquired = acquired

 try:
 for lock in locks:
 lock.acquire()
 yield
 finally:
 # Release locks in reverse order of acquisition
 for lock in reversed(locks):
 lock.release()
 del acquired[-len(locks):]

import threading
x_lock = threading.Lock()
y_lock = threading.Lock()

def thread_1():
 while True:
 with acquire(x_lock, y_lock):
 print('Thread-1')

def thread_2():
 while True:
 with acquire(y_lock, x_lock):
 print('Thread-2')

t1 = threading.Thread(target=thread_1)
t1.daemon = True
t1.start()

t2 = threading.Thread(target=thread_2)
t2.daemon = True
t2.start()

import threading
x_lock = threading.Lock()
y_lock = threading.Lock()

def thread_1():

 while True:
 with acquire(x_lock):
 with acquire(y_lock):
 print('Thread-1')

def thread_2():
 while True:
 with acquire(y_lock):
 with acquire(x_lock):
 print('Thread-2')

t1 = threading.Thread(target=thread_1)
t1.daemon = True
t1.start()

t2 = threading.Thread(target=thread_2)
t2.daemon = True
t2.start()

Exception in thread Thread-1:
Traceback (most recent call last):
 File "/usr/local/lib/python3.3/threading.py", line 639, in _bootstrap_inner
 self.run()
 File "/usr/local/lib/python3.3/threading.py", line 596, in run
 self._target(*self._args, **self._kwargs)
 File "deadlock.py", line 49, in thread_1
 with acquire(y_lock):
 File "/usr/local/lib/python3.3/contextlib.py", line 48, in __enter__
 return next(self.gen)
 File "deadlock.py", line 15, in acquire
 raise RuntimeError("Lock Order Violation")
RuntimeError: Lock Order Violation
>>>

import threading

# The philosopher thread
def philosopher(left, right):
 while True:
 with acquire(left,right):
 print(threading.currentThread(), 'eating')

# The chopsticks (represented by locks)
NSTICKS = 5
chopsticks = [threading.Lock() for n in range(NSTICKS)]

# Create all of the philosophers
for n in range(NSTICKS):
 t = threading.Thread(target=philosopher,
 args=(chopsticks[n],chopsticks[(n+1) % NSTICKS]))
 t.start()