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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"TSAN alarm in `dictobject.c` while using `setattr`","articleBody":"# Bug report\n\n### Bug description:\nWhile working around with my previous issue, I found another TSAN alarm.\n\n```python\nfrom threading import Thread\nimport random\nimport copy\n\nE = Exception()\n\ndef test1():\n    for _ in range(100):\n        setattr(E, 'x', random.randint(0,1000))\n        copy.copy(E)\n\ndef main():\n    threads = []\n    for _ in range(4):\n        threads.append(Thread(target=test1))\n\n    for t in threads:\n        t.start()\n\n    for t in threads:\n        t.join()\n\nif __name__ == \"__main__\":\n    main()\n```\n\nTSAN report:\n```\nWARNING: ThreadSanitizer: data race (pid=455134)\n  Read of size 8 at 0x7fffb60c01b0 by thread T1:\n    #0 _PyDict_Next /cpython/Objects/dictobject.c:3020:40 (python3.15t+0x291d09) \n    #1 PyDict_Next /cpython/Objects/dictobject.c:3074:12 (python3.15t+0x291d09)\n    #2 BaseException___setstate___impl /cpython/Objects/exceptions.c:245:16 (python3.15t+0x232246) \n    #3 BaseException___setstate__ /cpython/Objects/clinic/exceptions.c.h:48:20 (python3.15t+0x232246)\n    #4 method_vectorcall_O /cpython/Objects/descrobject.c:476:24 (python3.15t+0x2262d5) \n    #5 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x20c593) \n    #6 PyObject_Vectorcall /cpython/Objects/call.c:327:12 (python3.15t+0x20c593)\n    #7 _Py_VectorCallInstrumentation_StackRefSteal /cpython/Python/ceval.c:762:11 (python3.15t+0x422cbc) \n    #8 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:1817:35 (python3.15t+0x428ce6) \n    #9 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090) \n    #10 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)\n    #11 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f) \n    #12 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6) \n    #13 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)\n    #14 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x476281) \n    #15 context_run /cpython/Python/context.c:727:29 (python3.15t+0x476281)\n    #16 method_vectorcall_FASTCALL_KEYWORDS /cpython/Objects/descrobject.c:421:24 (python3.15t+0x226017) \n    #17 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x20c593) \n    #18 PyObject_Vectorcall /cpython/Objects/call.c:327:12 (python3.15t+0x20c593)\n    #19 _Py_VectorCallInstrumentation_StackRefSteal /cpython/Python/ceval.c:762:11 (python3.15t+0x422cbc) \n    #20 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:1817:35 (python3.15t+0x428ce6) \n    #21 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090) \n    #22 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)\n    #23 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f) \n    #24 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6) \n    #25 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)\n    #26 _PyVectorcall_Call /cpython/Objects/call.c:273:16 (python3.15t+0x20c89b) \n    #27 _PyObject_Call /cpython/Objects/call.c:348:16 (python3.15t+0x20c89b)\n    #28 PyObject_Call /cpython/Objects/call.c:373:12 (python3.15t+0x20c905) \n    #29 thread_run /cpython/./Modules/_threadmodule.c:387:21 (python3.15t+0x5f5512) \n    #30 pythread_wrapper /cpython/Python/thread_pthread.h:234:5 (python3.15t+0x526127) \n\n  Previous atomic write of size 8 at 0x7fffb60c01b0 by thread T2:\n    #0 _Py_atomic_store_ptr_release /cpython/./Include/cpython/pyatomic_gcc.h:565:3 (python3.15t+0x29435b) \n    #1 insertdict /cpython/Objects/dictobject.c (python3.15t+0x29435b)\n    #2 setitem_take2_lock_held /cpython/Objects/dictobject.c:2684:12 (python3.15t+0x2936a7) \n    #3 setitem_lock_held /cpython/Objects/dictobject.c:2721:12 (python3.15t+0x29ff83) \n    #4 _PyDict_SetItem_LockHeld /cpython/Objects/dictobject.c:6850:16 (python3.15t+0x29ff83)\n    #5 _PyObjectDict_SetItem /cpython/Objects/dictobject.c:7549:11 (python3.15t+0x2a3d97) \n    #6 _PyObject_GenericSetAttrWithDict /cpython/Objects/object.c:2055:19 (python3.15t+0x2cbf44) \n    #7 PyObject_GenericSetAttr /cpython/Objects/object.c:2083:12 (python3.15t+0x2cc697) \n    #8 PyObject_SetAttr /cpython/Objects/object.c:1528:15 (python3.15t+0x2c8f4f) \n    #9 builtin_setattr_impl /cpython/Python/bltinmodule.c:1744:9 (python3.15t+0x41d2e7) \n    #10 builtin_setattr /cpython/Python/clinic/bltinmodule.c.h:692:20 (python3.15t+0x41d2e7)\n    #11 cfunction_vectorcall_FASTCALL /cpython/Objects/methodobject.c:449:24 (python3.15t+0x2bbb1e) \n    #12 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x20c593) \n    #13 PyObject_Vectorcall /cpython/Objects/call.c:327:12 (python3.15t+0x20c593)\n    #14 _Py_VectorCallInstrumentation_StackRefSteal /cpython/Python/ceval.c:762:11 (python3.15t+0x422cbc) \n    #15 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:1817:35 (python3.15t+0x428ce6) \n    #16 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090) \n    #17 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)\n    #18 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f) \n    #19 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6) \n    #20 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)\n    #21 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x476281) \n    #22 context_run /cpython/Python/context.c:727:29 (python3.15t+0x476281)\n    #23 method_vectorcall_FASTCALL_KEYWORDS /cpython/Objects/descrobject.c:421:24 (python3.15t+0x226017) \n    #24 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x20c593) \n    #25 PyObject_Vectorcall /cpython/Objects/call.c:327:12 (python3.15t+0x20c593)\n    #26 _Py_VectorCallInstrumentation_StackRefSteal /cpython/Python/ceval.c:762:11 (python3.15t+0x422cbc) \n    #27 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:1817:35 (python3.15t+0x428ce6) \n    #28 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15t+0x422090) \n    #29 _PyEval_Vector /cpython/Python/ceval.c:2125:12 (python3.15t+0x422090)\n    #30 _PyFunction_Vectorcall /cpython/Objects/call.c (python3.15t+0x20cc0f) \n    #31 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:136:11 (python3.15t+0x2112e6) \n    #32 method_vectorcall /cpython/Objects/classobject.c:74:20 (python3.15t+0x2112e6)\n    #33 _PyVectorcall_Call /cpython/Objects/call.c:273:16 (python3.15t+0x20c89b) \n    #34 _PyObject_Call /cpython/Objects/call.c:348:16 (python3.15t+0x20c89b)\n    #35 PyObject_Call /cpython/Objects/call.c:373:12 (python3.15t+0x20c905) \n    #36 thread_run /cpython/./Modules/_threadmodule.c:387:21 (python3.15t+0x5f5512) \n    #37 pythread_wrapper /cpython/Python/thread_pthread.h:234:5 (python3.15t+0x526127) \n\nSUMMARY: ThreadSanitizer: data race /cpython/Objects/dictobject.c:3020:40 in _PyDict_Next\n```\nVersion: Python 3.15.0a5+ free-threading build (heads/main:d5f96c86653, Feb 11 2026, 04:50:14) [Clang 18.1.3 (1ubuntu1)]\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-144828\n* gh-150578\n* gh-153746\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/Naserume","@type":"Person","name":"Naserume"},"datePublished":"2026-02-13T02:57:44.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/144774/cpython/issues/144774"}