How to synchronize two alternating threads

Question

I need to start two threads, controlling which one starts first, then having them alternating their jobs.

The following code works as expected with do_sleep = True , but it can fail with do_sleep = False .

How can I achieve the same result without using those ugly (and unreliable) sleeps?

The reason why it works with do_sleep = True is that:

• Each worker thread gives time to the other thread to start before trying to acquire the lock and start the next job
• There is a pause between the start of the first and the second worker that allows the first one to acquire the lock before the second is ready

With do_sleep = False it can fail because:

• At the end of each job, each thread can try to acquire the lock for the next cycle before the other thread, executing two consecutive jobs instead of alternating
• The second thread could acquire the lock before the first one

Here is the code:

import threading
import time
import random

do_sleep = True

def workerA(lock):
    for i in range(5):
        lock.acquire()
        print('Working A - %s' % i)
        time.sleep(random.uniform(0.2, 1))
        lock.release()
        if do_sleep: time.sleep(0.1)

def workerB(lock):
    for i in range(5):
        if do_sleep: time.sleep(0.1)
        lock.acquire()
        print('Working B - %s' % i)
        time.sleep(random.uniform(0.2, 1))
        lock.release()
        if do_sleep: time.sleep(0.1)

lock = threading.Lock()

t1 = threading.Thread(target=workerA, args=(lock, ))
t2 = threading.Thread(target=workerB, args=(lock, ))

t1.start()
if do_sleep: time.sleep(0.1)
t2.start()

t1.join()
t2.join()

print('done')

EDIT Using a Queue as suggested by Mike doesn't help, because the first worker would finish the job without waiting for the second.

This is the wrong output of a version after replacing the Lock with a Queue :

Working A - 0
Working A - 1
Working B - 0
Working A - 2
Working B - 1
Working A - 3
Working B - 2
Working A - 4
Working B - 3
Working B - 4
done

This is the wrong output, obtained with do_sleep = False :

Working A - 0
Working A - 1
Working A - 2
Working A - 3
Working A - 4
Working B - 0
Working B - 1
Working B - 2
Working B - 3
Working B - 4
done

This is the correct output, obtained with do_sleep = True :

Working A - 0
Working B - 0
Working A - 1
Working B - 1
Working A - 2
Working B - 2
Working A - 3
Working B - 3
Working A - 4
Working B - 4
done

Answer 1

Several ways to solve this. One relatively easy one is to use the lock to control access to a separate shared variable: call this other variable owner , it can either be set to A or B. Thread A can only start a job when owner is set to A, and thread B can only start a job when owner is set to B. Then the pseudo-code is (assume thread A here):

while True:
    while True:
        # Loop until I'm the owner
        lock.acquire()
        if owner == A:
            break
        lock.release()

    # Now I'm the owner. And I still hold the lock. Start job.
    <Grab next job (or start job or finish job, whatever is required to remove it from contention)>
    owner = B
    lock.release()
    <Finish job if not already done. Go get next one>

The B thread does the same thing only reversing the if owner and owner = statements. And obviously you can parameterize it so that both actually just run the same code.

EDIT

Here is the working version, with the suggested logic inside an object:

import threading
import time

def workerA(lock):
    for i in range(5):
        lock.acquire_for('A')
        print('Start A - %s' % i)
        time.sleep(0.5)
        print('End A - %s' % i)
        lock.release_to('B')

def workerB(lock):
    for i in range(5):
        lock.acquire_for('B')
        print('Start B - %s' % i)
        time.sleep(2)
        print('End B - %s' % i)
        lock.release_to('A')

class LockWithOwner:

    lock = threading.RLock()
    owner = 'A'

    def acquire_for(self, owner):
        n = 0
        while True:
            self.lock.acquire()
            if self.owner == owner:
                break
            n += 1
            self.lock.release()
            time.sleep(0.001)
        print('Waited for {} to be the owner {} times'.format(owner, n))

    def release_to(self, new_owner):
        self.owner = new_owner
        self.lock.release()

lock = LockWithOwner()
lock.owner = 'A'

t1 = threading.Thread(target=workerA, args=(lock, ))
t2 = threading.Thread(target=workerB, args=(lock, ))

t1.start()
t2.start()

t1.join()
t2.join()

print('done')

Answer 2

You can exclude the possibility of the wrong thread acquiring the lock, exclude relying on time.sleep(...) for correctness and shorten your code at the same time using Queue (two queues for both way communication):

import threading
import time
import random
from Queue import Queue

def work_hard(name, i):
  print('start %s - %s' % (name, i))
  time.sleep(random.uniform(0.2, 1))
  print('end %s - %s' % (name, i))

def worker(name, q_mine, q_his):
  for i in range(5):
    q_mine.get()
    work_hard(name, i)
    q_his.put(1)

qAB = Queue()
qBA = Queue()

t1 = threading.Thread(target=worker, args=('A', qAB, qBA))
t2 = threading.Thread(target=worker, args=('B', qBA, qAB))

t1.start()
qAB.put(1) # notice how you don't need time.sleep(...) even here
t2.start()

t1.join()
t2.join()

print('done')

It works as you specified. Alternatively you can use threading.Condition (a combination of acquire , release , wait and notify / notifyAll ), but that will be more subtle, especially in terms of which thread goes first.

Answer 3

I have tried Gil Hamilton's answer and it doesn't work for me if I remove all the sleeps. I think it's because my 'main' thread keeps getting the priority. I found out that a better way to synchronize two or more threads is to use conditional object.

Here is my working alternate lock object with conditional object inside

class AltLock():
    def __init__(self, initial_thread):
        self.allow = initial_thread
        self.cond = threading.Condition()
        
    def acquire_for(self, thread):
        self.cond.acquire()
        while self.allow!=thread:
            print("\tLOCK:", thread, "waiting")
            self.cond.wait()
        print("\tLOCK:", thread, "acquired")
    
    def release_to(self, thread):
        print("\tLOCK: releasing to", thread)
        self.allow=thread
        self.cond.notifyAll()
        self.cond.release()

And this is an example usecase (the sleep statements in the thread are not required):

class MyClass():
    def __init__(self):
        self.lock = AltLock("main")
        
    def _start(self):
        print("thread: Started, wait 2 second")
        time.sleep(2)
        print("---")
        self.lock.acquire_for("thread")
        time.sleep(2)
        print("---")
        print("thread: start lock acquired")
        self.lock.release_to("main")
        return 0
    
    def start(self):
        self.lock.acquire_for("main")
        self.thread = threading.Thread(target = self._start, )
        self.thread.start()
        print("main: releasing lock")
        self.lock.release_to("thread")
        self.lock.acquire_for("main")
        print("main: lock acquired")

myclass = MyClass()
myclass.start()
myclass.lock.release_to("main") # house keeping

And this is stdout:

    LOCK: main acquired
thread: Started, wait 2 second
main: releasing lock
    LOCK: releasing to thread
    LOCK: main waiting         // 'main' thread try to reacquire the lock immediately but get blocked by wait.
---
    LOCK: thread acquired
---
thread: start lock acquired
    LOCK: releasing to main
    LOCK: main acquired
main: lock acquired
    LOCK: releasing to main