How to solve racing conditions in python using shared memory

racing.py

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