Why does this Python code with threading have race conditions?

Question

This code creates a race condition:

import threading

ITERS = 100000
x = [0]

def worker():
    for _ in range(ITERS):
        x[0] += 1  # this line creates a race condition
        # because it takes a value, increments and then writes
        # some inrcements can be done together, and lost

def main():
    x[0] = 0  # you may use `global x` instead of this list trick too
    t1 = threading.Thread(target=worker)
    t2 = threading.Thread(target=worker)
    t1.start()
    t2.start()
    t1.join()
    t2.join()

for i in range(5):
    main()
    print(f'iteration {i}. expected x = {ITERS*2}, got {x[0]}')

Output:

$ python3 test.py
iteration 0. expected x = 200000, got 200000
iteration 1. expected x = 200000, got 148115
iteration 2. expected x = 200000, got 155071
iteration 3. expected x = 200000, got 200000
iteration 4. expected x = 200000, got 200000

Python3 version:

Python 3.9.7 (default, Sep 10 2021, 14:59:43) 
[GCC 11.2.0] on linux

I thought GIL would prevent it and not allow two threads run together until they do something io-related or call a C library. At least this is what you may conclude from the docs.

Then, what does GIL actually do, and when do threads run in parallel?

Answer 1

Reading the docs better, I think there's the answer:

The mechanism used by the CPython interpreter to assure that only one thread executes Python bytecode at a time. This simplifies the CPython implementation by making the object model (including critical built-in types such as dict) implicitly safe against concurrent access. Locking the entire interpreter makes it easier for the interpreter to be multi-threaded, at the expense of much of the parallelism afforded by multi-processor machines.

However, some extension modules, either standard or third-party, are designed so as to release the GIL when doing computationally-intensive tasks such as compression or hashing. Also, the GIL is always released when doing I/O.

I don't know the internals, but guess each line or block of this bytecode is executed alone, and other threads are waiting (which makes it slow). But some lines consist of multiple blocks, and aren't atomic.

Here's what you get if run dis.dis('x[0] += 1'):

          0 LOAD_NAME                0 (x)
          2 LOAD_CONST               0 (0)
          4 DUP_TOP_TWO
          6 BINARY_SUBSCR
          8 LOAD_CONST               1 (1)
         10 INPLACE_ADD
         12 ROT_THREE
         14 STORE_SUBSCR
         16 LOAD_CONST               2 (None)
         18 RETURN_VALUE

Some of these are executed in concurrent way, and make the race condition. So GIL only guarantees the internals of structures like list or dict won't be damaged.

Answer 2

As per our final comments, it appears as though this has been fixed (ubuntu, windows) with python version 3.10 and above. This issue is no longer experienced.

However, there other scenarios where race conditions can be obsevered. For example this:

import threading
import time
 
x = 10
 
def increment(by):
    global x
 
    local_counter = x
    local_counter += by
 
    time.sleep(1)
 
    x = local_counter
    print(f'{threading.current_thread().name} inc x {by}, x: {x}')
 
def main():
    # creating threads
    t1 = threading.Thread(target=increment, args=(5,))
    t2 = threading.Thread(target=increment, args=(10,))
   
    # starting the threads
    t1.start()
    t2.start()
   
    # waiting for the threads to complete
    t1.join()
    t2.join()
   
    print(f'The final value of x is {x}')
 
for i in range(10):
    main()

which produces this:

Thread-56 (increment) inc x 10, x: 20Thread-55 (increment) inc x 5, x: 15
 
The final value of x is 15
Thread-57 (increment) inc x 5, x: 20Thread-58 (increment) inc x 10, x: 25
 
The final value of x is 25
Thread-60 (increment) inc x 10, x: 35Thread-59 (increment) inc x 5, x: 30
 
The final value of x is 30
Thread-61 (increment) inc x 5, x: 35
Thread-62 (increment) inc x 10, x: 40
The final value of x is 40
Thread-64 (increment) inc x 10, x: 50Thread-63 (increment) inc x 5, x: 45
 
The final value of x is 45

but the fix here is to use the asyncio module to control the flow of the code.