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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Data race in `_PyImport_SetDLOpenFlags` when `sys.setdlopenflags()` is called concurrently","articleBody":"# Bug report\n\n### Bug description:\n\nIn the free-threaded build, calling `sys.setdlopenflags()` concurrently from multiple threads (or concurrently with `sys.getdlopenflags()`) data-races on the per-interpreter `dlopenflags` field: `_PyImport_GetDLOpenFlags` / `_PyImport_SetDLOpenFlags` read and write `(interp)-\u003eimports.dlopenflags` with no atomics or lock.\n\nhttps://github.com/python/cpython/blob/9e863fab283eddca9c2a8f9d1ee30f4dc243e314/Python/import.c#L908-L918\n\n`sys.setdlopenflags()` / `sys.getdlopenflags()` expose these directly, so concurrent calls produce a non-atomic write/write (and read/write) race on the field.\n\nReproducer:\n```python\nimport sys\nfrom threading import Thread, Barrier\n\nN = 12\nbarrier = Barrier(N)\n\ndef worker():\n    barrier.wait()\n    for _ in range(20000):\n        d = sys.getdlopenflags()\n        sys.setdlopenflags(d)\n\nif __name__ == \"__main__\":\n    threads = [Thread(target=worker) for _ in range(N)]\n    for t in threads: t.start()\n    for t in threads: t.join()\n```\n\nTSAN Report:\n```\n==================\nWARNING: ThreadSanitizer: data race (pid=2296999)\n  Read of size 4 at 0x555555e3f648 by thread T1:\n    #0 _PyImport_GetDLOpenFlags /cpython/Python/import.c:911:12 \n    #1 sys_getdlopenflags_impl /cpython/./Python/sysmodule.c:1838:13 \n    #2 sys_getdlopenflags /cpython/./Python/clinic/sysmodule.c.h:823:12 \n    #3 cfunction_vectorcall_NOARGS /cpython/Objects/methodobject.c:508:24 \n    #4 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:144:11 \n    #5 PyObject_Vectorcall /cpython/Objects/call.c:327:12 \n    #6 _Py_VectorCallInstrumentation_StackRefSteal /cpython/Python/ceval.c:768:11 \n    #7 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:1846:35 \n    ...\n\n  Previous write of size 4 at 0x555555e3f648 by thread T12:\n    #0 _PyImport_SetDLOpenFlags /cpython/Python/import.c:917:25 \n    #1 sys_setdlopenflags_impl /cpython/./Python/sysmodule.c:1819:5 \n    #2 sys_setdlopenflags /cpython/./Python/clinic/sysmodule.c.h:796:20 \n    #3 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:2612:35 \n    ...\n\n  Location is global '_PyRuntime' of size 405824 at 0x555555e16c80 \n\nSUMMARY: ThreadSanitizer: data race /cpython/Python/import.c:911:12 in _PyImport_GetDLOpenFlags\n==================\n==================\nWARNING: ThreadSanitizer: data race (pid=2296999)\n  Write of size 4 at 0x555555e3f648 by thread T12:\n    #0 _PyImport_SetDLOpenFlags /cpython/Python/import.c:917:25 \n    #1 sys_setdlopenflags_impl /cpython/./Python/sysmodule.c:1819:5 \n    #2 sys_setdlopenflags /cpython/./Python/clinic/sysmodule.c.h:796:20 \n    #3 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:2612:35 \n    ...\n\n  Previous write of size 4 at 0x555555e3f648 by thread T9:\n    #0 _PyImport_SetDLOpenFlags /cpython/Python/import.c:917:25 \n    #1 sys_setdlopenflags_impl /cpython/./Python/sysmodule.c:1819:5 \n    #2 sys_setdlopenflags /cpython/./Python/clinic/sysmodule.c.h:796:20 \n    #3 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:2612:35 \n    ...\n\n  Location is global '_PyRuntime' of size 405824 at 0x555555e16c80 \n\nSUMMARY: ThreadSanitizer: data race /cpython/Python/import.c:917:25 in _PyImport_SetDLOpenFlags\n==================\n```\n\n### CPython versions tested on:\n\n3.16\n\n### Operating systems tested on:\n\nLinux\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-151768\n* gh-153789\n* gh-153790\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/Naserume","@type":"Person","name":"Naserume"},"datePublished":"2026-06-18T11:09:01.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/151644/cpython/issues/151644"}