Progress on No-GIL CPython

Re:Progress on No-GIL CPython by pdonis on Saturday October 21, 2023 @10:12PM (#37972250)

But this isn't actually true; that was my point. Neither of these operations, if they are part of an actual statement or expression, will complete in a single byte code. That means that, with the GIL, the operations can be interrupted with a thread switch between bytecodes. So, for example, a statement like

a += b

takes four bytecodes: two LOAD bytecodes to put the values of a and b on the stack, an INPLACE_ADD bytecode to put the result at the top of the stack, and a STORE bytecode to store the result in the variable a. A thread switch could take place in between any two of these bytecodes, and if another thread mutated a or b or was reading the value of a or b, you would have data corruption. The GIL does not prevent this. The only way to prevent it would be to use one of the locking mechanisms provided to guard access to the a and b variables so that only one thread could access them at a time.

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Re:Progress on No-GIL CPython by shrimpx on Monday October 23, 2023 @02:19AM (#37982181)

Thank you, I believe you are right. I did try to make a race condition happen using += and am having a hard time. Here's my code

    import threading

    i = 0

    def test():
        global i
        for x in range(1000000):
            i += 1


    threads = [threading.Thread(target=test) for t in range(20)]
    for t in threads:
        t.start()

    for t in threads:
        t.join()

    assert i == 20000000, i

Presumably the assert should fail sometimes but I haven't been able to observe that.

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