René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Race in `Thread.join()`","articleBody":"# Bug report\n\n### Bug description:\n\nThere is a race between when `Thread._tstate_lock` is released[^1] in `Thread._wait_for_tstate_lock()` and when `Thread._stop()` asserts[^2] that it is unlocked. Consider the following execution involving threads A, B, and C:\n\n1. A starts.\n2. B joins A, blocking on its `_tstate_lock`.\n3. C joins A, blocking on its `_tstate_lock`.\n4. A finishes and releases its `_tstate_lock`.\n5. B acquires A's `_tstate_lock` in `_wait_for_tstate_lock()`, releases it, but is swapped out before calling `_stop()`.\n6. C is scheduled, acquires A's `_tstate_lock` in `_wait_for_tstate_lock()` but is swapped out before releasing it.\n7. B is scheduled, calls `_stop()`, which asserts that A's `_tstate_lock` is not held. However, C holds it, so the assertion fails.\n\nThe race can be reproduced[^3] by inserting sleeps at the appropriate points in the threading code. To do so, run the `repro_join_race.py` from the linked repo.\n\n[^1]: https://github.com/python/cpython/blob/441affc9e7f419ef0b68f734505fa2f79fe653c7/Lib/threading.py#L1201\n[^2]: https://github.com/python/cpython/blob/441affc9e7f419ef0b68f734505fa2f79fe653c7/Lib/threading.py#L1115\n[^3]: https://github.com/mpage/cpython/commit/81946532792f938cd6f6ab4c4ff92a4edf61314f\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux","author":{"url":"https://github.com/mpage","@type":"Person","name":"mpage"},"datePublished":"2024-03-05T17:31:26.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/116372/cpython/issues/116372"}