Readers & Writers problem - thread safe i python.

from threading import Thread
from threading import Lock
import time

resource = time.strftime("%c")
lock = Lock()
counter = 0

class WriterA(Thread):
  def run(self):
  while True:
    global lock
    global resource
    global counter
      if counter == 0:
        lock.acquire()
        counter == -4
        resource = time.strftime("%c")
        print ("Writer A acquired the lock and the message is: %s. " %resource)
        counter = 0
        lock.release()
        time.sleep(5)

class WriterB(Thread):
	def run(self):
		while True:
			global lock
			global resource
			global counter
			if counter == 0:
				lock.acquire()
				counter == -5
				resource = resource[::-1]
				print ("WriterB acquired the lock and the message is: %s. " %resource)
				counter = 0
				lock.release()
				time.sleep(5)

class Reader(Thread):
        def run(self):
                while True:
                        global resource
                        global counter
                        if counter >= 0:
                                counter = 1
                                print ("Reader acquired the resource and the message is: %s" %resource)
                                counter -= 1
                                time.sleep(5)

                                

writerA = WriterA()
writerB = WriterB()
reader1 = Reader()
reader2 = Reader()
reader3 = Reader()

writerA.start()
writerB.start()
reader1.start()
reader2.start()
reader3.start()