为什么这个 Python 线程代码有竞争条件？

Question

这段代码创建了一个竞争条件：

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]}')

输出：

$ 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版本：

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

我认为 GIL 会阻止它并且不允许两个线程运行在一起，直到它们执行与 io 相关的操作或调用 C 库。至少这是你可以从 the docs.

得出的结论

那么，GIL到底做了什么，线程什么时候运行并行？

Answer 1

阅读 the docs 更好，我认为有答案：

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.

我不知道内部原理，但猜测这个字节码的每一行或每一块都是单独执行的，其他线程正在等待（这使得它很慢）。但是有些行由多个块组成，不是原子的。

如果运行 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

其中一些以并发方式执行，并产生竞争条件。所以GIL只保证像list或dict这样的结构内部不会被破坏。

为什么这个 Python 线程代码有竞争条件？

Why does this Python code with threading have race conditions?

python

gil

python-3.x