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May 23, 2014 10:28
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How to solve racing conditions in python using shared memory
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import time | |
from threading import Thread | |
import random | |
d1 = {} | |
def run(n): | |
for i in range(3): | |
c = d1['c'] | |
print('Thread %s read c=%s' % (n, c)) | |
time.sleep(1) | |
c += 1 | |
d1['c'] = c | |
print('Thread %s wrote c=%s' % (n, c)) | |
time.sleep(random.random()) | |
def racing(): | |
d1['c'] = 0 | |
t1 = Thread(target=run, args=(1,)) | |
t2 = Thread(target=run, args=(2,)) | |
t1.start() | |
t2.start() | |
t1.join() | |
t2.join() | |
c = d1['c'] | |
print('Finish. c = %s' % c) | |
memcache = {} | |
def waitforlock(name, timeout=0): | |
start = time.time() | |
locked = memcache.get(name) | |
def should_wait_for_timeout(): | |
return timeout <= 0 or time.time() - start < timeout | |
while(locked and should_wait_for_timeout()): | |
time.sleep(0.1) | |
locked = memcache.get(name) | |
if not locked: | |
memcache[name] = True | |
return not locked | |
def releaselock(name): | |
del memcache[name] | |
class lockon(): | |
def __init__(self, name): | |
self.name = name | |
def __enter__(self): | |
gotlock = waitforlock(self.name) | |
if not gotlock: | |
raise BaseException('cannot get lock for %s' % self.name) | |
def __exit__(self, type, value, traceback): | |
releaselock(self.name) | |
def controlled_run(n): | |
for i in range(3): | |
with lockon('write_to_c'): | |
c = d1['c'] | |
print('Thread %s read c=%s' % (n, c)) | |
time.sleep(1) | |
c += 1 | |
d1['c'] = c | |
print('Thread %s wrote c=%s' % (n, c)) | |
time.sleep(random.random()) | |
def racing_solved(): | |
d1['c'] = 0 | |
t1 = Thread(target=controlled_run, args=(1,)) | |
t2 = Thread(target=controlled_run, args=(2,)) | |
t1.start() | |
t2.start() | |
t1.join() | |
t2.join() | |
c = d1['c'] | |
print('Finish. c = %s' % c) | |
racing(); # hum, this is not good | |
racing_solved(); # now, this works |
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