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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ThreadSanitizer: data race in _struct module initialization with InterpreterPoolExecutor (free-threading build)","articleBody":"# Bug report\n\n### Bug description:\n\nThreadSanitizer detects a data race when using `InterpreterPoolExecutor` with the free-threading build. Multiple threads simultaneously write to the global variable `lilendian_table` in the `_struct` module during module initialization.\n\nThe race occurs at:\n- **Thread T1 and T2**: Both write to `lilendian_table` at `_structmodule_exec` (lines 2745:31 and 2746:33)\n- **Location**: `global 'lilendian_table' of size 840 at 0x7ffff4513ca0` in `_struct.cpython-315td-x86_64-linux-gnu.so`\n\nBoth threads are executing through `InterpreterPoolExecutor`, importing modules in separate subinterpreters, which triggers concurrent initialization of the `_struct` extension module.\n\n### How to Reproduce\n\n**Build configuration:**\n```bash\nCC=clang CXX=clang++ ./configure --with-thread-sanitizer --with-pydebug --enable-experimental-jit=yes --with-lto=full --with-tail-call-interp --disable-gil\nmake -j\n```\n\n**Run command:**\n```bash\nTSAN_OPTIONS=handle_segv=0 ./python -X dev -X showrefcount bug.py\n```\n\n**Test script (bug.py):**\n```python\nfrom concurrent.futures import InterpreterPoolExecutor\nfrom functools import lru_cache\n\n@lru_cache(maxsize=512)\ndef fib(n):\n    \"\"\"Function that JIT will compile\"\"\"\n    if n \u003c 2:\n        return n\n    return fib(n-1) + fib(n-2)\n\nif __name__ == '__main__':\n    with InterpreterPoolExecutor() as t:\n        for i in range(2):\n            t.submit(fib, i)\n```\n\n**Note:** The race occurs with any loop iteration count (range(1), range(2), etc.)\n\n### TSAN Output\n\n[log.txt](https://github.com/user-attachments/files/22975650/log.txt)\n\n### Environment\n\n- **CPython version**: main branch\n- **Operating System**: Linux\n- **Compiler**: Clang\n- **Build flags**: `--with-thread-sanitizer --with-pydebug --enable-experimental-jit=yes --with-lto=full --with-tail-call-interp --disable-gil`\n\n### Additional Information\n\nA second similar data race is also reported for another write in `_structmodule_exec` at line 2746:33 to the same global variable.\n\nBoth races involve `InterpreterPoolExecutor` threads executing module initialization code concurrently in different subinterpreters.\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-140909\n* gh-141501\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/ashm-dev","@type":"Person","name":"ashm-dev"},"datePublished":"2025-10-17T16:20:20.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/140260/cpython/issues/140260"}