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GeezerVison is showing github.com — Data race in set iterator length_hint under no-gil · Issue #144356 · python/cpython

Data race in set iterator length_hint under no-gil #144356

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Data race in set iterator length_hint under no-gil#144356
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interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)topic-free-threadingtype-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

Description

@hyongtao-code
hyongtao-code
opened on Jan 31, 2026
Issue body actions

Bug report

Build info

$ cd cpython-tsan/bin/
$ ./python3.15
Python 3.15.0a5+ free-threading build (heads/main:96e4cd698a, Jan 31 2026, 11:45:37) [Clang 18.1.3 (1ubuntu1)] on linux
Type "help", "copyright", "credits" or "license" for more information.

Bug description

Under Py_GIL_DISABLED, setiter_len() reads so->used non-atomically while concurrent set mutations update it atomically.

This triggers a data race detectable by TSAN when one thread repeatedly calls iterator.__length_hint__() while another mutates the same set

Python script

import threading, time

s = set(range(2000))
it = iter(s)
stop = threading.Event()

def reader():
    while not stop.is_set():
        it.__length_hint__()

def writer():
    i = 0
    while not stop.is_set():
        s.add(i)
        s.discard(i - 1)
        i += 1

threads = [threading.Thread(target=reader) for _ in range(4)]
threads.append(threading.Thread(target=writer))

for t in threads: t.start()
time.sleep(3)
stop.set()
for t in threads: t.join()

TSAN result

Details 
$ TSAN_OPTIONS="halt_on_error=1:history_size=7:report_signal_unsafe=0:verbosity=1" ./python3.15 ./tsan_setiter_len_race.py 
==1703386==WARNING: ThreadSanitizer: memory layout is incompatible, possibly due to high-entropy ASLR.
Re-execing with fixed virtual address space.
N.B. reducing ASLR entropy is preferable.
==1703386==Installed the sigaction for signal 11
==1703386==Installed the sigaction for signal 7
==1703386==Installed the sigaction for signal 8
***** Running under ThreadSanitizer v3 (pid 1703386) *****
==================
WARNING: ThreadSanitizer: data race (pid=1703386)
  Atomic write of size 8 at 0x7fffb4395ce8 by thread T5:
    #0 _Py_atomic_store_ssize_relaxed /home/hyongtao/worksapce/cpython/./Include/cpython/pyatomic_gcc.h:513:3 (python3.15+0x30378c) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 set_discard_entry /home/hyongtao/worksapce/cpython/Objects/setobject.c:594:5 (python3.15+0x30378c)
    #2 set_discard_key /home/hyongtao/worksapce/cpython/Objects/setobject.c:630:12 (python3.15+0x30378c)
    #3 set_discard_impl /home/hyongtao/worksapce/cpython/Objects/setobject.c:2616:10 (python3.15+0x30b3c3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 set_discard /home/hyongtao/worksapce/cpython/Objects/clinic/setobject.c.h:502:20 (python3.15+0x30b3c3)
    #5 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:4033:35 (python3.15+0x42abe4) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #8 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #10 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #11 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x472d31) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 context_run /home/hyongtao/worksapce/cpython/Python/context.c:727:29 (python3.15+0x472d31)
    #13 method_vectorcall_FASTCALL_KEYWORDS /home/hyongtao/worksapce/cpython/Objects/descrobject.c:421:24 (python3.15+0x224ab7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #14 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20b1e3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #15 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b1e3)
    #16 _Py_VectorCallInstrumentation_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:762:11 (python3.15+0x4204bc) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #17 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:1809:35 (python3.15+0x426376) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #19 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #20 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #22 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #23 _PyVectorcall_Call /home/hyongtao/worksapce/cpython/Objects/call.c:273:16 (python3.15+0x20b4eb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #24 _PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:348:16 (python3.15+0x20b4eb)
    #25 PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:373:12 (python3.15+0x20b555) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 thread_run /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:387:21 (python3.15+0x5f12e2) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #27 pythread_wrapper /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:234:5 (python3.15+0x522317) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Previous read of size 8 at 0x7fffb4395ce8 by thread T3:
    #0 setiter_len /home/hyongtao/worksapce/cpython/Objects/setobject.c:1059:58 (python3.15+0x304ce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3935:35 (python3.15+0x42a9d4) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #4 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #7 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x472d31) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 context_run /home/hyongtao/worksapce/cpython/Python/context.c:727:29 (python3.15+0x472d31)
    #9 method_vectorcall_FASTCALL_KEYWORDS /home/hyongtao/worksapce/cpython/Objects/descrobject.c:421:24 (python3.15+0x224ab7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #10 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20b1e3) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b1e3)
    #12 _Py_VectorCallInstrumentation_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:762:11 (python3.15+0x4204bc) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:1809:35 (python3.15+0x426376) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #14 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f890) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #15 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f890)
    #16 _PyFunction_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c (python3.15+0x20b85f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #17 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:136:11 (python3.15+0x20ff36) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 method_vectorcall /home/hyongtao/worksapce/cpython/Objects/classobject.c:73:20 (python3.15+0x20ff36)
    #19 _PyVectorcall_Call /home/hyongtao/worksapce/cpython/Objects/call.c:273:16 (python3.15+0x20b4eb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 _PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:348:16 (python3.15+0x20b4eb)
    #21 PyObject_Call /home/hyongtao/worksapce/cpython/Objects/call.c:373:12 (python3.15+0x20b555) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #22 thread_run /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:387:21 (python3.15+0x5f12e2) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 pythread_wrapper /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:234:5 (python3.15+0x522317) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Thread T5 'Thread-5 (write' (tid=1703392, running) created by main thread at:
    #0 pthread_create <null> (python3.15+0xf421f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 do_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:281:14 (python3.15+0x521424) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:323:9 (python3.15+0x52123a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 ThreadHandle_start /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:474:9 (python3.15+0x5f109a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 do_start_new_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:1920:9 (python3.15+0x5f0b3f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 thread_PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:2043:14 (python3.15+0x5efc01) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 cfunction_call /home/hyongtao/worksapce/cpython/Objects/methodobject.c:564:18 (python3.15+0x2bab87) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyObject_MakeTpCall /home/hyongtao/worksapce/cpython/Objects/call.c:242:18 (python3.15+0x20a68f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:134:16 (python3.15+0x20b2a7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b2a7)
    #10 _Py_VectorCall_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:720:11 (python3.15+0x41fce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3387:35 (python3.15+0x429c79) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f3b0) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f3b0)
    #14 PyEval_EvalCode /home/hyongtao/worksapce/cpython/Python/ceval.c:673:21 (python3.15+0x41f3b0)
    #15 run_eval_code_obj /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1366:12 (python3.15+0x4fe967) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #16 run_mod /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1469:19 (python3.15+0x4fe967)
    #17 pyrun_file /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1294:15 (python3.15+0x4f9d13) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyRun_SimpleFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:518:13 (python3.15+0x4f9d13)
    #19 _PyRun_AnyFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:81:15 (python3.15+0x4f9498) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 pymain_run_file_obj /home/hyongtao/worksapce/cpython/Modules/main.c:410:15 (python3.15+0x53bedf) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 pymain_run_file /home/hyongtao/worksapce/cpython/Modules/main.c:429:15 (python3.15+0x53bedf)
    #22 pymain_run_python /home/hyongtao/worksapce/cpython/Modules/main.c:691:21 (python3.15+0x53b20b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 Py_RunMain /home/hyongtao/worksapce/cpython/Modules/main.c:772:5 (python3.15+0x53b20b)
    #24 pymain_main /home/hyongtao/worksapce/cpython/Modules/main.c:802:12 (python3.15+0x53b778) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #25 Py_BytesMain /home/hyongtao/worksapce/cpython/Modules/main.c:826:12 (python3.15+0x53b7fb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 main /home/hyongtao/worksapce/cpython/./Programs/python.c:15:12 (python3.15+0x17282b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

  Thread T3 'Thread-3 (reade' (tid=1703390, running) created by main thread at:
    #0 pthread_create <null> (python3.15+0xf421f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #1 do_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:281:14 (python3.15+0x521424) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #2 PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/Python/thread_pthread.h:323:9 (python3.15+0x52123a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #3 ThreadHandle_start /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:474:9 (python3.15+0x5f109a) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #4 do_start_new_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:1920:9 (python3.15+0x5f0b3f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #5 thread_PyThread_start_joinable_thread /home/hyongtao/worksapce/cpython/./Modules/_threadmodule.c:2043:14 (python3.15+0x5efc01) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #6 cfunction_call /home/hyongtao/worksapce/cpython/Objects/methodobject.c:564:18 (python3.15+0x2bab87) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #7 _PyObject_MakeTpCall /home/hyongtao/worksapce/cpython/Objects/call.c:242:18 (python3.15+0x20a68f) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #8 _PyObject_VectorcallTstate /home/hyongtao/worksapce/cpython/./Include/internal/pycore_call.h:134:16 (python3.15+0x20b2a7) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #9 PyObject_Vectorcall /home/hyongtao/worksapce/cpython/Objects/call.c:327:12 (python3.15+0x20b2a7)
    #10 _Py_VectorCall_StackRefSteal /home/hyongtao/worksapce/cpython/Python/ceval.c:720:11 (python3.15+0x41fce9) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #11 _PyEval_EvalFrameDefault /home/hyongtao/worksapce/cpython/Python/generated_cases.c.h:3387:35 (python3.15+0x429c79) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #12 _PyEval_EvalFrame /home/hyongtao/worksapce/cpython/./Include/internal/pycore_ceval.h:118:16 (python3.15+0x41f3b0) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #13 _PyEval_Vector /home/hyongtao/worksapce/cpython/Python/ceval.c:2092:12 (python3.15+0x41f3b0)
    #14 PyEval_EvalCode /home/hyongtao/worksapce/cpython/Python/ceval.c:673:21 (python3.15+0x41f3b0)
    #15 run_eval_code_obj /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1366:12 (python3.15+0x4fe967) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #16 run_mod /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1469:19 (python3.15+0x4fe967)
    #17 pyrun_file /home/hyongtao/worksapce/cpython/Python/pythonrun.c:1294:15 (python3.15+0x4f9d13) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #18 _PyRun_SimpleFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:518:13 (python3.15+0x4f9d13)
    #19 _PyRun_AnyFileObject /home/hyongtao/worksapce/cpython/Python/pythonrun.c:81:15 (python3.15+0x4f9498) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #20 pymain_run_file_obj /home/hyongtao/worksapce/cpython/Modules/main.c:410:15 (python3.15+0x53bedf) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #21 pymain_run_file /home/hyongtao/worksapce/cpython/Modules/main.c:429:15 (python3.15+0x53bedf)
    #22 pymain_run_python /home/hyongtao/worksapce/cpython/Modules/main.c:691:21 (python3.15+0x53b20b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #23 Py_RunMain /home/hyongtao/worksapce/cpython/Modules/main.c:772:5 (python3.15+0x53b20b)
    #24 pymain_main /home/hyongtao/worksapce/cpython/Modules/main.c:802:12 (python3.15+0x53b778) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #25 Py_BytesMain /home/hyongtao/worksapce/cpython/Modules/main.c:826:12 (python3.15+0x53b7fb) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)
    #26 main /home/hyongtao/worksapce/cpython/./Programs/python.c:15:12 (python3.15+0x17282b) (BuildId: 00135b3b874b2f13c2b74122623dcba7b1b9ce4a)

SUMMARY: ThreadSanitizer: data race /home/hyongtao/worksapce/cpython/./Include/cpython/pyatomic_gcc.h:513:3 in _Py_atomic_store_ssize_relaxed
==================

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

  • gh-144356: Make set iterator __length_hint__ and iternext race-safe under no-gil #144357

Activity

  1. hyongtao-code
    added
    type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error
    on Jan 31, 2026
  2. aisk
    added
    interpreter-core(Objects, Python, Grammar, and Parser dirs)(Objects, Python, Grammar, and Parser dirs)
    topic-free-threading
    on Jan 31, 2026

  3. devdanzin commented on Jul 19, 2026

    @devdanzin
    devdanzin
    on Jul 19, 2026
    Member
    More actions

    fusil --tsan independently hit this, via a different trigger than the report's __length_hint__-vs-set-mutation script: sharing a single set iterator across threads (no set mutation at all). setiter_iternext reads si->si_pos inside Py_BEGIN_CRITICAL_SECTION(so) (setobject.c:1117) but writes it back outside the section (si->si_pos = i+1, :1128), and si->len-- is likewise unsynchronized — so two threads advancing the same iterator race on its private cursor. setiter_len reading si->len races the same write. The section is keyed on the set so, so it doesn't serialize concurrent use of one iterator.

    import operator, threading

NTHREADS = 8
barrier = threading.Barrier(NTHREADS)

def worker(it):
    barrier.wait()
    for _ in range(20000):
        try:
            next(it)                    # setiter_iternext: si->si_pos / si->len advance
        except StopIteration:
            pass
        operator.length_hint(it, 0)     # setiter_len: reads si->len

for _ in range(300):
    shared = iter(set(range(4096)))     # ONE shared iterator, no set mutation
    ts = [threading.Thread(target=worker, args=(shared,)) for _ in range(NTHREADS)]
    for t in ts: t.start()
    for t in ts: t.join()

    Exit 66 under TSan; setiter_iternext:1117 (read si->si_pos) vs :1128 (write), plus the setiter_len | setiter_iternext (si->len) face. This is exactly the "concurrent use of the same iterator" case PR #144357 addresses by switching to Py_BEGIN_CRITICAL_SECTION2(self, so) and moving the cursor writes inside the section — so it's a useful corroboration of that expanded scope. Confirmed on current main (3.16.0a0), debug + release TSan builds.

    (Found by fusil --tsan, a ThreadSanitizer fuzzer; draft and reproducer by Claude Code, minimized and reviewed by hand.)

  4. devdanzin commented on Jul 20, 2026

    @devdanzin
    devdanzin
    on Jul 20, 2026
    Member
    More actions

    This isn't only a __length_hint__ data race — the same setiter_iternext path double-frees the set and crashes. On a free-threaded build, setiter_iternext (Objects/setobject.c) reads so = si->si_set unguarded, takes only the set's critical section around the table scan, and then on exhaustion runs — outside that section:

        if (key == NULL) {
        si->si_set = NULL;      /* :1130 */
        Py_DECREF(so);          /* :1131  drop the iterator's one owning ref to the set */
        return NULL;
    }

    Two threads advancing the same set iterator to exhaustion both read the same non-NULL so and both Py_DECREF(so) → the set's refcount underflows → use-after-free.

    Reproducer:

    import threading

NT = 8
def newit():
    return iter(set(range(32)))
cell = [newit()]
def worker():
    for _ in range(200_000):
        it = cell[0]
        try:
            next(it)
        except StopIteration:
            cell[0] = newit()
        except Exception:
            pass
ts = [threading.Thread(target=worker) for _ in range(NT)]
for t in ts: t.start()
for t in ts: t.join()

    PYTHON_GIL=0 ./python repro.py:

    • debug free-threaded build → Objects/setobject.c:1131: _Py_NegativeRefcount: object has negative ref count, ~8/8 within seconds.
    • release -O0 free-threaded build (no sanitizer) → SIGSEGV / SIGABRT, core dumped, 6/6.

    The realistic form uses a long-lived shared frozenset (e.g. a module-level constant, refcount > 1): the underflow is silent until the next gc.collect() reports Python/gc_free_threading.c:999 update_refs: Assertion "refcount >= 0" failed on a frozenset with a wild refcount (~2^60), or a later access UAFs.

    PR #144357 fixes this — widening to Py_BEGIN_CRITICAL_SECTION2(self, so) and, under Py_GIL_DISABLED, making exhaustion sticky via si_pos = -1 and removing the si_set = NULL; Py_DECREF(so) from iternext (dropping the set ref only in dealloc) is exactly what closes the double-DECREF. It'd be good to land it — this is a memory-safety bug, not just a value race. (This is the set sibling of the dict-iterator double-free just filed as gh-154130.)

    (Found by fusil --tsan, a ThreadSanitizer fuzzer; crash confirmed by re-running the reproducer without a sanitizer on plain free-threaded builds. Comment + reproducer by Claude Code, reviewed by hand.)

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