René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Data race between `record_allocation` and `gc_get_count_impl`","articleBody":"# Bug report\n\n### Bug description:\n\nI found this during solve #148613.\n\nThe `gc_generation.count` has one read not protected by atomic operation.\n\n```python\nimport gc\nfrom threading import Barrier, Thread\n\nclass CyclicReference:\n    def __init__(self):\n        self.ref = self\n\nN_ALLOCATORS = 7\nN_READERS = 1\nN_THREADS = N_ALLOCATORS + N_READERS\nN_ITERS = 200_000\n\nbarrier = Barrier(N_THREADS)\n\ndef allocator():\n    barrier.wait()\n    for _ in range(N_ITERS):\n        CyclicReference()\n\n\ndef reader():\n    barrier.wait()\n    for _ in range(N_ITERS):\n        gc.get_count()\n\nthreads = [Thread(target=allocator) for _ in range(N_ALLOCATORS)]\nthreads.extend(Thread(target=reader) for _ in range(N_READERS))\n\nfor thread in threads:\n    thread.start()\nfor thread in threads:\n    thread.join()\n```\n\nHere is tsan report.\n\n```\n==================\nWARNING: ThreadSanitizer: data race (pid=2274087)\n  Atomic write of size 4 at 0x55a842f6ab04 by thread T2:\n    #0 _Py_atomic_add_int Include/cpython/pyatomic_gcc.h:15 (python3.16+0x45a310)\n    #1 record_allocation Python/gc_free_threading.c:2028 (python3.16+0x45a310)\n    #2 _PyObject_GC_Link Python/gc_free_threading.c:2707 (python3.16+0x45a310)\n    #3 _PyType_AllocNoTrack Objects/typeobject.c:2533 (python3.16+0x299867)\n    #4 PyType_GenericAlloc Objects/typeobject.c:2554 (python3.16+0x2998d4)\n    #5 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1955 (python3.16+0x3e0047)\n    #6 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #7 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #8 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #9 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #10 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #11 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #12 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42927b)\n    #13 context_run Python/context.c:728 (python3.16+0x42927b)\n    #14 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x174575)\n    #15 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #16 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #17 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:775 (python3.16+0x3c13f5)\n    #18 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cee3f)\n    #19 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #20 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #21 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #22 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #23 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #24 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #25 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154c78)\n    #26 _PyObject_Call Objects/call.c:348 (python3.16+0x1550b1)\n    #27 PyObject_Call Objects/call.c:373 (python3.16+0x1550b1)\n    #28 thread_run Modules/_threadmodule.c:388 (python3.16+0x609d30)\n    #29 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x508c52)\n    #30 \u003cnull\u003e \u003cnull\u003e (libtsan.so.2+0x4cf1a)\n\n  Previous read of size 4 at 0x55a842f6ab04 by thread T8:\n    #0 gc_get_count_impl Modules/gcmodule.c:232 (python3.16+0x52aef1)\n    #1 gc_get_count Modules/clinic/gcmodule.c.h:305 (python3.16+0x52aef1)\n    #2 cfunction_vectorcall_NOARGS Objects/methodobject.c:508 (python3.16+0x21a586)\n    #3 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #4 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #5 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #6 _PyEval_EvalFrameDefault Python/generated_cases.c.h:4362 (python3.16+0x3cf022)\n    #7 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #8 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #9 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #10 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #11 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #12 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #13 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42927b)\n    #14 context_run Python/context.c:728 (python3.16+0x42927b)\n    #15 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x174575)\n    #16 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #17 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #18 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:775 (python3.16+0x3c13f5)\n    #19 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cee3f)\n    #20 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #21 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #22 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #23 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #24 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #25 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #26 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154c78)\n    #27 _PyObject_Call Objects/call.c:348 (python3.16+0x1550b1)\n    #28 PyObject_Call Objects/call.c:373 (python3.16+0x1550b1)\n    #29 thread_run Modules/_threadmodule.c:388 (python3.16+0x609d30)\n    #30 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x508c52)\n    #31 \u003cnull\u003e \u003cnull\u003e (libtsan.so.2+0x4cf1a)\n\n  Location is global '_PyRuntime' of size 405696 at 0x55a842f42240 (python3.16+0x89fb04)\n\n  Thread T2 'Thread-2 (alloc' (tid=2274090, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (libtsan.so.2+0x58491)\n    #1 do_start_joinable_thread Python/thread_pthread.h:281 (python3.16+0x508d67)\n    #2 PyThread_start_joinable_thread Python/thread_pthread.h:323 (python3.16+0x5090ab)\n    #3 ThreadHandle_start Modules/_threadmodule.c:475 (python3.16+0x60aca9)\n    #4 do_start_new_thread Modules/_threadmodule.c:1919 (python3.16+0x60aca9)\n    #5 thread_PyThread_start_joinable_thread Modules/_threadmodule.c:2042 (python3.16+0x60b5b9)\n    #6 cfunction_call Objects/methodobject.c:564 (python3.16+0x2192fd)\n    #7 _PyObject_MakeTpCall Objects/call.c:242 (python3.16+0x150e5b)\n    #8 _PyObject_VectorcallTstate Include/internal/pycore_call.h:142 (python3.16+0x151de1)\n    #9 _PyObject_VectorcallTstate Include/internal/pycore_call.h:129 (python3.16+0x151de1)\n    #10 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151de1)\n    #11 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #12 _PyEval_EvalFrameDefault Python/generated_cases.c.h:3528 (python3.16+0x3d8f55)\n    #13 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee06c)\n    #14 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee06c)\n    #15 PyEval_EvalCode Python/ceval.c:686 (python3.16+0x3ee06c)\n    #16 run_eval_code_obj Python/pythonrun.c:1369 (python3.16+0x4d55c2)\n    #17 run_mod Python/pythonrun.c:1472 (python3.16+0x4d5b88)\n    #18 pyrun_file Python/pythonrun.c:1296 (python3.16+0x4d9913)\n    #19 _PyRun_SimpleFileObject Python/pythonrun.c:518 (python3.16+0x4d9913)\n    #20 _PyRun_AnyFileObject Python/pythonrun.c:81 (python3.16+0x4da3c5)\n    #21 pymain_run_file_obj Modules/main.c:411 (python3.16+0x52940d)\n    #22 pymain_run_file Modules/main.c:430 (python3.16+0x52940d)\n    #23 pymain_run_python Modules/main.c:715 (python3.16+0x52940d)\n    #24 Py_RunMain Modules/main.c:796 (python3.16+0x52a2fd)\n    #25 pymain_main Modules/main.c:826 (python3.16+0x52a2fd)\n    #26 Py_BytesMain Modules/main.c:850 (python3.16+0x52a2fd)\n    #27 main Programs/python.c:15 (python3.16+0xa0786)\n\n  Thread T8 'Thread-8 (reade' (tid=2274096, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (libtsan.so.2+0x58491)\n    #1 do_start_joinable_thread Python/thread_pthread.h:281 (python3.16+0x508d67)\n    #2 PyThread_start_joinable_thread Python/thread_pthread.h:323 (python3.16+0x5090ab)\n    #3 ThreadHandle_start Modules/_threadmodule.c:475 (python3.16+0x60aca9)\n    #4 do_start_new_thread Modules/_threadmodule.c:1919 (python3.16+0x60aca9)\n    #5 thread_PyThread_start_joinable_thread Modules/_threadmodule.c:2042 (python3.16+0x60b5b9)\n    #6 cfunction_call Objects/methodobject.c:564 (python3.16+0x2192fd)\n    #7 _PyObject_MakeTpCall Objects/call.c:242 (python3.16+0x150e5b)\n    #8 _PyObject_VectorcallTstate Include/internal/pycore_call.h:142 (python3.16+0x151de1)\n    #9 _PyObject_VectorcallTstate Include/internal/pycore_call.h:129 (python3.16+0x151de1)\n    #10 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151de1)\n    #11 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #12 _PyEval_EvalFrameDefault Python/generated_cases.c.h:3528 (python3.16+0x3d8f55)\n    #13 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee06c)\n    #14 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee06c)\n    #15 PyEval_EvalCode Python/ceval.c:686 (python3.16+0x3ee06c)\n    #16 run_eval_code_obj Python/pythonrun.c:1369 (python3.16+0x4d55c2)\n    #17 run_mod Python/pythonrun.c:1472 (python3.16+0x4d5b88)\n    #18 pyrun_file Python/pythonrun.c:1296 (python3.16+0x4d9913)\n    #19 _PyRun_SimpleFileObject Python/pythonrun.c:518 (python3.16+0x4d9913)\n    #20 _PyRun_AnyFileObject Python/pythonrun.c:81 (python3.16+0x4da3c5)\n    #21 pymain_run_file_obj Modules/main.c:411 (python3.16+0x52940d)\n    #22 pymain_run_file Modules/main.c:430 (python3.16+0x52940d)\n    #23 pymain_run_python Modules/main.c:715 (python3.16+0x52940d)\n    #24 Py_RunMain Modules/main.c:796 (python3.16+0x52a2fd)\n    #25 pymain_main Modules/main.c:826 (python3.16+0x52a2fd)\n    #26 Py_BytesMain Modules/main.c:850 (python3.16+0x52a2fd)\n    #27 main Programs/python.c:15 (python3.16+0xa0786)\n\nSUMMARY: ThreadSanitizer: data race Include/cpython/pyatomic_gcc.h:15 in _Py_atomic_add_int\n==================\n==================\nWARNING: ThreadSanitizer: data race (pid=2274087)\n  Atomic write of size 4 at 0x55a842f6ab04 by thread T3:\n    #0 _Py_atomic_compare_exchange_int Include/cpython/pyatomic_gcc.h:70 (python3.16+0x45b47f)\n    #1 record_deallocation Python/gc_free_threading.c:2057 (python3.16+0x45b47f)\n    #2 PyObject_GC_Del Python/gc_free_threading.c:2831 (python3.16+0x45b47f)\n    #3 object_dealloc Objects/typeobject.c:7463 (python3.16+0x286bab)\n    #4 subtype_dealloc Objects/typeobject.c:2871 (python3.16+0x28e8b7)\n    #5 _Py_Dealloc Objects/object.c:3312 (python3.16+0x22599b)\n    #6 _Py_DecRefSharedDebug Objects/object.c:426 (python3.16+0x225d5d)\n    #7 _Py_DecRefShared Objects/object.c:433 (python3.16+0x225d5d)\n    #8 Py_DECREF Include/refcount.h:385 (python3.16+0x45ca4c)\n    #9 delete_garbage Python/gc_free_threading.c:1769 (python3.16+0x45ca4c)\n    #10 gc_collect_internal Python/gc_free_threading.c:2176 (python3.16+0x45ca4c)\n    #11 gc_collect_main Python/gc_free_threading.c:2257 (python3.16+0x45d5d6)\n    #12 _Py_RunGC Python/gc_free_threading.c:2716 (python3.16+0x45dbb6)\n    #13 _Py_HandlePending Python/ceval_gil.c:1399 (python3.16+0x46d393)\n    #14 check_periodics Python/ceval_macros.h:524 (python3.16+0x3e9e88)\n    #15 _PyEval_EvalFrameDefault Python/generated_cases.c.h:10876 (python3.16+0x3e9e88)\n    #16 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #17 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #18 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #19 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #20 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #21 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #22 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42927b)\n    #23 context_run Python/context.c:728 (python3.16+0x42927b)\n    #24 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x174575)\n    #25 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #26 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #27 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:775 (python3.16+0x3c13f5)\n    #28 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cee3f)\n    #29 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #30 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #31 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #32 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #33 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #34 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #35 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154c78)\n    #36 _PyObject_Call Objects/call.c:348 (python3.16+0x1550b1)\n    #37 PyObject_Call Objects/call.c:373 (python3.16+0x1550b1)\n    #38 thread_run Modules/_threadmodule.c:388 (python3.16+0x609d30)\n    #39 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x508c52)\n    #40 \u003cnull\u003e \u003cnull\u003e (libtsan.so.2+0x4cf1a)\n\n  Previous read of size 4 at 0x55a842f6ab04 by thread T8:\n    #0 gc_get_count_impl Modules/gcmodule.c:232 (python3.16+0x52aef1)\n    #1 gc_get_count Modules/clinic/gcmodule.c.h:305 (python3.16+0x52aef1)\n    #2 cfunction_vectorcall_NOARGS Objects/methodobject.c:508 (python3.16+0x21a586)\n    #3 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #4 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #5 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #6 _PyEval_EvalFrameDefault Python/generated_cases.c.h:4362 (python3.16+0x3cf022)\n    #7 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #8 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #9 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #10 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #11 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #12 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #13 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x42927b)\n    #14 context_run Python/context.c:728 (python3.16+0x42927b)\n    #15 method_vectorcall_FASTCALL_KEYWORDS Objects/descrobject.c:421 (python3.16+0x174575)\n    #16 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x151d7c)\n    #17 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151d7c)\n    #18 _Py_VectorCallInstrumentation_StackRefSteal Python/ceval.c:775 (python3.16+0x3c13f5)\n    #19 _PyEval_EvalFrameDefault Python/generated_cases.c.h:1846 (python3.16+0x3cee3f)\n    #20 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee7b7)\n    #21 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee7b7)\n    #22 _PyFunction_Vectorcall Objects/call.c:413 (python3.16+0x152190)\n    #23 _PyObject_VectorcallTstate Include/internal/pycore_call.h:144 (python3.16+0x15379f)\n    #24 _PyObject_VectorcallPrepend Objects/call.c:855 (python3.16+0x15379f)\n    #25 method_vectorcall Objects/classobject.c:55 (python3.16+0x158676)\n    #26 _PyVectorcall_Call Objects/call.c:273 (python3.16+0x154c78)\n    #27 _PyObject_Call Objects/call.c:348 (python3.16+0x1550b1)\n    #28 PyObject_Call Objects/call.c:373 (python3.16+0x1550b1)\n    #29 thread_run Modules/_threadmodule.c:388 (python3.16+0x609d30)\n    #30 pythread_wrapper Python/thread_pthread.h:234 (python3.16+0x508c52)\n    #31 \u003cnull\u003e \u003cnull\u003e (libtsan.so.2+0x4cf1a)\n\n  Location is global '_PyRuntime' of size 405696 at 0x55a842f42240 (python3.16+0x89fb04)\n\n  Thread T3 'Thread-3 (alloc' (tid=2274091, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (libtsan.so.2+0x58491)\n    #1 do_start_joinable_thread Python/thread_pthread.h:281 (python3.16+0x508d67)\n    #2 PyThread_start_joinable_thread Python/thread_pthread.h:323 (python3.16+0x5090ab)\n    #3 ThreadHandle_start Modules/_threadmodule.c:475 (python3.16+0x60aca9)\n    #4 do_start_new_thread Modules/_threadmodule.c:1919 (python3.16+0x60aca9)\n    #5 thread_PyThread_start_joinable_thread Modules/_threadmodule.c:2042 (python3.16+0x60b5b9)\n    #6 cfunction_call Objects/methodobject.c:564 (python3.16+0x2192fd)\n    #7 _PyObject_MakeTpCall Objects/call.c:242 (python3.16+0x150e5b)\n    #8 _PyObject_VectorcallTstate Include/internal/pycore_call.h:142 (python3.16+0x151de1)\n    #9 _PyObject_VectorcallTstate Include/internal/pycore_call.h:129 (python3.16+0x151de1)\n    #10 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151de1)\n    #11 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #12 _PyEval_EvalFrameDefault Python/generated_cases.c.h:3528 (python3.16+0x3d8f55)\n    #13 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee06c)\n    #14 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee06c)\n    #15 PyEval_EvalCode Python/ceval.c:686 (python3.16+0x3ee06c)\n    #16 run_eval_code_obj Python/pythonrun.c:1369 (python3.16+0x4d55c2)\n    #17 run_mod Python/pythonrun.c:1472 (python3.16+0x4d5b88)\n    #18 pyrun_file Python/pythonrun.c:1296 (python3.16+0x4d9913)\n    #19 _PyRun_SimpleFileObject Python/pythonrun.c:518 (python3.16+0x4d9913)\n    #20 _PyRun_AnyFileObject Python/pythonrun.c:81 (python3.16+0x4da3c5)\n    #21 pymain_run_file_obj Modules/main.c:411 (python3.16+0x52940d)\n    #22 pymain_run_file Modules/main.c:430 (python3.16+0x52940d)\n    #23 pymain_run_python Modules/main.c:715 (python3.16+0x52940d)\n    #24 Py_RunMain Modules/main.c:796 (python3.16+0x52a2fd)\n    #25 pymain_main Modules/main.c:826 (python3.16+0x52a2fd)\n    #26 Py_BytesMain Modules/main.c:850 (python3.16+0x52a2fd)\n    #27 main Programs/python.c:15 (python3.16+0xa0786)\n\n  Thread T8 'Thread-8 (reade' (tid=2274096, running) created by main thread at:\n    #0 pthread_create \u003cnull\u003e (libtsan.so.2+0x58491)\n    #1 do_start_joinable_thread Python/thread_pthread.h:281 (python3.16+0x508d67)\n    #2 PyThread_start_joinable_thread Python/thread_pthread.h:323 (python3.16+0x5090ab)\n    #3 ThreadHandle_start Modules/_threadmodule.c:475 (python3.16+0x60aca9)\n    #4 do_start_new_thread Modules/_threadmodule.c:1919 (python3.16+0x60aca9)\n    #5 thread_PyThread_start_joinable_thread Modules/_threadmodule.c:2042 (python3.16+0x60b5b9)\n    #6 cfunction_call Objects/methodobject.c:564 (python3.16+0x2192fd)\n    #7 _PyObject_MakeTpCall Objects/call.c:242 (python3.16+0x150e5b)\n    #8 _PyObject_VectorcallTstate Include/internal/pycore_call.h:142 (python3.16+0x151de1)\n    #9 _PyObject_VectorcallTstate Include/internal/pycore_call.h:129 (python3.16+0x151de1)\n    #10 PyObject_Vectorcall Objects/call.c:327 (python3.16+0x151de1)\n    #11 _Py_VectorCall_StackRefSteal Python/ceval.c:733 (python3.16+0x3c0fc5)\n    #12 _PyEval_EvalFrameDefault Python/generated_cases.c.h:3528 (python3.16+0x3d8f55)\n    #13 _PyEval_EvalFrame Include/internal/pycore_ceval.h:122 (python3.16+0x3ee06c)\n    #14 _PyEval_Vector Python/ceval.c:2156 (python3.16+0x3ee06c)\n    #15 PyEval_EvalCode Python/ceval.c:686 (python3.16+0x3ee06c)\n    #16 run_eval_code_obj Python/pythonrun.c:1369 (python3.16+0x4d55c2)\n    #17 run_mod Python/pythonrun.c:1472 (python3.16+0x4d5b88)\n    #18 pyrun_file Python/pythonrun.c:1296 (python3.16+0x4d9913)\n    #19 _PyRun_SimpleFileObject Python/pythonrun.c:518 (python3.16+0x4d9913)\n    #20 _PyRun_AnyFileObject Python/pythonrun.c:81 (python3.16+0x4da3c5)\n    #21 pymain_run_file_obj Modules/main.c:411 (python3.16+0x52940d)\n    #22 pymain_run_file Modules/main.c:430 (python3.16+0x52940d)\n    #23 pymain_run_python Modules/main.c:715 (python3.16+0x52940d)\n    #24 Py_RunMain Modules/main.c:796 (python3.16+0x52a2fd)\n    #25 pymain_main Modules/main.c:826 (python3.16+0x52a2fd)\n    #26 Py_BytesMain Modules/main.c:850 (python3.16+0x52a2fd)\n    #27 main Programs/python.c:15 (python3.16+0xa0786)\n\nSUMMARY: ThreadSanitizer: data race Include/cpython/pyatomic_gcc.h:70 in _Py_atomic_compare_exchange_int\n==================\nThreadSanitizer: reported 2 warnings\n```\n\n## Root Cause\n\nThis problem would happen when cyclic references allocation frequently in free threading build.\n\nThe `alloc_count` would be reset when `gc-\u003ealloc_count \u003e= LOCAL_ALLOC_COUNT_THRESHOLD` and the `gcstate-\u003eyoung.count` would increase.\nhttps://github.com/python/cpython/blob/c714b5679817099ac574890392f408129e6c67cb/Python/gc_free_threading.c#L2025-L2029\n\nAnd we read the count in `gc_get_count_impl`, it was partially protected by atomic.\nhttps://github.com/python/cpython/blob/c714b5679817099ac574890392f408129e6c67cb/Modules/gcmodule.c#L211-L237\n\n## Solution\n\nSo we should add atomic load for the read action.\n\nFor `count` in `gc_collect_internal`, they are guarded by STW so I think they should be safe.\nhttps://github.com/python/cpython/blob/c714b5679817099ac574890392f408129e6c67cb/Python/gc_free_threading.c#L2065-L2077\n\n\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-150413\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/LindaSummer","@type":"Person","name":"LindaSummer"},"datePublished":"2026-05-25T16:03:50.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/150411/cpython/issues/150411"}