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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"An interpreter's initial thread can be accessed while finalizing","articleBody":"# Crash report\n\n### What happened?\n\nWhile working on gh-126644, I came across an elusive bug that seems to occur when a lot of threads are trying to create a thread state for the same interpreter. Initially, I thought it was an issue with free-threading, but it occurs on the default build as well.\n\nHere's a small reproducer:\n\n```py\nfrom threading import Thread\nimport _interpreters\n\ninterp = _interpreters.create()\n\ndef run():\n    this_interp = _interpreters.create()\n    _interpreters.run_string(this_interp, f\"import _interpreters; _interpreters.run_string({interp}, '1')\")\n\nthreads = [Thread(target=run) for _ in range(1000)]\nfor thread in threads:\n    thread.start()\n\nfor thread in threads:\n    thread.join()\n```\n\nAs it turns out, this is because of this check: https://github.com/python/cpython/blob/main/Python/pystate.c#L1531\n\nInside `tstate_delete_common`, the runtime lock is released after the `threads.head` has already been set to `NULL`, so other threads think erroneously think it's OK to try and use `_initial_thread` while it's still getting finalized. Possibly related, `free_threadstate` tries to reset `_initial_thread` back to the default settings without the runtime lock, which is possibly racy.\n\nThere's a few ways to fix this, but I think the easiest (and has the least amount of risk for backporting) is to just hold the runtime lock for all of thread state deletion. This will introduce a very slight slowdown due to lock contention, but that should get better once #114940 lands. I have a PR ready.\n\n### CPython versions tested on:\n\n3.12, 3.13, 3.14, CPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n### Output from running 'python -VV' on the command line:\n\n_No response_\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-126915\n* gh-126989\n* gh-127114\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ZeroIntensity","@type":"Person","name":"ZeroIntensity"},"datePublished":"2024-11-16T22:41:33.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/126914/cpython/issues/126914"}