Os Thread Race Condition

In this tutorial, you'll learn about How to Fix Thread Race Condition Errors. We cover key concepts, practical examples, and best practices.

Fix thread race condition errors when shared variable increment without lock causes data loss.

Quick Fix

Wrong

import threading
cnt=0
def worker():
    global cnt
    for _ in range(100000): cnt+=1  # Not atomic!
ts=[threading.Thread(target=worker) for _ in range(10)]
[t.start() for t in ts]; [t.join() for t in ts]
print(cnt)  # < 1000000 (lost updates)

cnt expected 1000000 but due to race condition gets ~500000-900000. Lost updates from non-atomic +=.

Right

import threading
lock=threading.Lock(); cnt=0
def worker():
    global cnt
    for _ in range(100000):
        with lock: cnt+=1
ts=[threading.Thread(target=worker) for _ in range(10)]
[t.start() for t in ts]; [t.join() for t in ts]
print(cnt)  # 1000000

1000000. Lock ensures mutual exclusion. One thread increments at a time.

Prevention

Use Lock for shared mutable state. Prefer threading primitives over relying on GIL.

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