gh-151722: Defer GC tracking of frozendict to end of construction by corona10 · Pull Request #151740 · python/cpython

corona10 · 2026-06-19T15:36:20Z

Issue: Data race on frozendict reads during construction in the free-threading build #151722

corona10 · 2026-06-19T15:43:03Z

TSAN script

import threading, gc

N = 4000
stop = threading.Event()
box = []

def reader():
    while not stop.is_set():
        if box:
            o = box[0]
            try:
                len(o); hash(o); repr(o)
            except Exception:
                pass

class Evil:
    def __init__(self):
        self.n = 0
    def keys(self):
        return [f"k{i}" for i in range(N)]
    def __getitem__(self, k):
        self.n += 1
        if self.n == 10 and not box:        # one scan/construction; k0..k8 present
            for o in gc.get_objects():
                if type(o) is frozendict and "k0" in o and len(o) < N:
                    box.append(o)
                    break
        return 1

t = threading.Thread(target=reader, daemon=True)
t.start()
for _ in range(40):
    box.clear()
    frozendict(Evil())
stop.set()
t.join(timeout=2)
print("observed half-built:", bool(box))

AS-IS

➜  cpython git:(main) ✗ TSAN_OPTIONS="halt_on_error=0" ./python.exe repro_151722.py
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Atomic write of size 8 at 0x00012a0e63d8 by main thread:
    #0 insert_combined_dict dictobject.c:1906 (python.exe:arm64+0x1000cca38)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)

  Previous read of size 8 at 0x00012a0e63d8 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be214)
    #1 frozendict_hash dictobject.c:8339 (python.exe:arm64+0x1000cbef4)
    #2 builtin_hash bltinmodule.c:1869 (python.exe:arm64+0x1001c792c)
    #3 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:1906 in insert_combined_dict
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a0e6488 by thread T1:
    #0 _PyDict_Next dictobject.c:3171 (python.exe:arm64+0x1000be260)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a0e6488 by main thread:
    #0 insert_combined_dict dictobject.c (python.exe:arm64+0x1000cc9e8)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3171 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Atomic write of size 8 at 0x00012a74ff80 by main thread:
    #0 dictresize dictobject.c:2279 (python.exe:arm64+0x1000cd9b4)
    #1 insert_combined_dict dictobject.c:1881 (python.exe:arm64+0x1000cc84c)
    #2 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #3 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #4 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #5 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #6 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)

  Previous read of size 8 at 0x00012a74ff80 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be208)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:2279 in dictresize
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a0e6480 by thread T1:
    #0 _PyDict_Next dictobject.c:3177 (python.exe:arm64+0x1000be2b8)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a0e6480 by main thread:
    #0 insert_combined_dict dictobject.c:1895 (python.exe:arm64+0x1000cc9a0)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3177 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a81c018 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be214)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a81c018 by main thread:
    #0 insert_combined_dict dictobject.c:1906 (python.exe:arm64+0x1000cca38)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3166 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Atomic write of size 8 at 0x00012a0e6918 by main thread:
    #0 insert_combined_dict dictobject.c:1906 (python.exe:arm64+0x1000cca38)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Previous read of size 8 at 0x00012a0e6918 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be214)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:1906 in insert_combined_dict
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Atomic write of size 8 at 0x00012a74ffc0 by main thread:
    #0 insertdict dictobject.c:2008 (python.exe:arm64+0x1000bcba4)
    #1 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #2 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #3 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #4 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Previous read of size 8 at 0x00012a74ffc0 by thread T1:
    #0 frozendict_length dictobject.c:3701 (python.exe:arm64+0x1000d243c)
    #1 _PyEval_EvalFrameDefault generated_cases.c.h:3880 (python.exe:arm64+0x1001d1d98)
    #2 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:2008 in insertdict
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a0e69c8 by thread T1:
    #0 _PyDict_Next dictobject.c:3171 (python.exe:arm64+0x1000be260)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a0e69c8 by main thread:
    #0 insert_combined_dict dictobject.c (python.exe:arm64+0x1000cc9e8)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3171 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Atomic write of size 8 at 0x00012a74ffd0 by main thread:
    #0 dictresize dictobject.c:2279 (python.exe:arm64+0x1000cd9b4)
    #1 insert_combined_dict dictobject.c:1881 (python.exe:arm64+0x1000cc84c)
    #2 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #3 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #4 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #5 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #6 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Previous read of size 8 at 0x00012a74ffd0 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be208)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:2279 in dictresize
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a0e69c0 by thread T1:
    #0 _PyDict_Next dictobject.c:3177 (python.exe:arm64+0x1000be2b8)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a0e69c0 by main thread:
    #0 insert_combined_dict dictobject.c:1895 (python.exe:arm64+0x1000cc9a0)
    #1 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #2 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #3 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #4 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3177 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a0ff198 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be214)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce448)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous write of size 8 at 0x00012a0ff198 by main thread:
    #0 new_keys_object dictobject.c:853 (python.exe:arm64+0x1000cc6d4)
    #1 dictresize dictobject.c:2174 (python.exe:arm64+0x1000cccd0)
    #2 insert_combined_dict dictobject.c:1881 (python.exe:arm64+0x1000cc84c)
    #3 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #4 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #5 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #6 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #7 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3166 in _PyDict_Next
==================
==================
WARNING: ThreadSanitizer: data race (pid=56255)
  Read of size 8 at 0x00012a108118 by thread T1:
    #0 _PyDict_Next dictobject.c:3166 (python.exe:arm64+0x1000be214)
    #1 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #2 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #3 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #4 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)

  Previous atomic write of size 8 at 0x00012a108118 by main thread:
    #0 dictresize dictobject.c:2292 (python.exe:arm64+0x1000cdb28)
    #1 insert_combined_dict dictobject.c:1881 (python.exe:arm64+0x1000cc84c)
    #2 insertdict dictobject.c:2005 (python.exe:arm64+0x1000bcb84)
    #3 setitem_take2_lock_held dictobject.c:2785 (python.exe:arm64+0x1000bbf38)
    #4 dict_merge dictobject.c:4280 (python.exe:arm64+0x1000bfd00)
    #5 frozendict_vectorcall dictobject.c:5331 (python.exe:arm64+0x1000cc248)
    #6 _PyEval_EvalFrameDefault generated_cases.c.h:2418 (python.exe:arm64+0x1001d0048)

  Thread T1 (tid=4632788, running) created by main thread at:
    #0 pthread_create <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x33834)
    #1 do_start_joinable_thread thread_pthread.h:281 (python.exe:arm64+0x100279b6c)
    #2 do_start_new_thread _threadmodule.c:1919 (python.exe:arm64+0x100332da4)
    #3 thread_PyThread_start_joinable_thread _threadmodule.c:2042 (python.exe:arm64+0x100331f0c)
    #4 cfunction_call methodobject.c:564 (python.exe:arm64+0x1000de0b8)

SUMMARY: ThreadSanitizer: data race dictobject.c:3166 in _PyDict_Next
==================
ThreadSanitizer:DEADLYSIGNAL
==56255==ERROR: ThreadSanitizer: SEGV on unknown address 0x000000000029 (pc 0x00010106faec bp 0x000170b8a540 sp 0x000170b8a500 T4632788)
==56255==The signal is caused by a READ memory access.
==56255==Hint: address points to the zero page.
    #0 __tsan_atomic64_load <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x5faec)
    #1 _PyDict_Next dictobject.c (python.exe:arm64+0x1000be2c8)
    #2 anydict_repr_impl dictobject.c:3621 (python.exe:arm64+0x1000ce668)
    #3 frozendict_repr dictobject.c:8271 (python.exe:arm64+0x1000cbdb0)
    #4 PyObject_Repr object.c:784 (python.exe:arm64+0x1000e46a0)
    #5 _PyEval_EvalFrameDefault generated_cases.c.h:2712 (python.exe:arm64+0x1001d07c4)
    #6 _PyEval_Vector ceval.c:2141 (python.exe:arm64+0x1001cc028)
    #7 _PyObject_VectorcallPrepend call.c:855 (python.exe:arm64+0x100063290)
    #8 context_run context.c:728 (python.exe:arm64+0x1001fff94)
    #9 PyObject_Vectorcall call.c:327 (python.exe:arm64+0x100061c10)
    #10 _Py_VectorCallInstrumentation_StackRefSteal ceval.c:768 (python.exe:arm64+0x1001cc64c)
    #11 _PyEval_EvalFrameDefault generated_cases.c.h:1906 (python.exe:arm64+0x1001cf74c)
    #12 _PyEval_Vector ceval.c:2141 (python.exe:arm64+0x1001cc028)
    #13 _PyObject_VectorcallPrepend call.c:855 (python.exe:arm64+0x100063290)
    #14 thread_run _threadmodule.c:388 (python.exe:arm64+0x100333580)
    #15 pythread_wrapper thread_pthread.h:234 (python.exe:arm64+0x10027a4ec)
    #16 __tsan_thread_start_func <null> (libclang_rt.tsan_osx_dynamic.dylib:arm64e+0x337a0)
    #17 _pthread_start <null> (libsystem_pthread.dylib:arm64e+0x6c54)
    #18 thread_start <null> (libsystem_pthread.dylib:arm64e+0x1c18)

==56255==Register values:
 x[0] = 0x0000000101000000   x[1] = 0x0000100000000050   x[2] = 0x00000000cf7d1ffe   x[3] = 0x0000000000000008
 x[4] = 0x0000000000000003   x[5] = 0x0000000000000000   x[6] = 0x0000000000000000   x[7] = 0x0000000000000000
 x[8] = 0x0000000000000000   x[9] = 0x000000000000001f  x[10] = 0x00000000c0000000  x[11] = 0x0000000000000000
x[12] = 0x000000010b485628  x[13] = 0x0000100000000050  x[14] = 0x0000000000000003  x[15] = 0x0000000000004010
x[16] = 0x0000000000000000  x[17] = 0x00000001010c4a40  x[18] = 0x0000000000000000  x[19] = 0x0000000000000029
x[20] = 0x0000000101000000  x[21] = 0x00000001003c62cc  x[22] = 0x0000000000000000  x[23] = 0x0000000000000001
x[24] = 0x0000000000000000  x[25] = 0x00000000000007ee  x[26] = 0x0000000000010707  x[27] = 0x000000012ace6240
x[28] = 0x000000012c0e0190     fp = 0x0000000170b8a540     lr = 0x000000010106fadc     sp = 0x0000000170b8a500
ThreadSanitizer can not provide additional info.
SUMMARY: ThreadSanitizer: SEGV dictobject.c in _PyDict_Next
==56255==ABORTING
[1]    56255 abort      TSAN_OPTIONS="halt_on_error=0" ./python.exe repro_151722.py

TO-BE

➜  cpython git:(gh-151722) ✗ TSAN_OPTIONS="halt_on_error=0" ./python.exe repro_1
51722.py
observed half-built: False

corona10 · 2026-06-19T16:28:10Z

cc @tonghuaroot

pythongh-151722: Defer GC tracking of frozendict to end of construction

2808df9

corona10 requested review from markshannon and methane as code owners June 19, 2026 15:36

bedevere-app Bot mentioned this pull request Jun 19, 2026

Data race on frozendict reads during construction in the free-threading build #151722

Open

bedevere-app Bot added the awaiting core review label Jun 19, 2026

corona10 requested review from colesbury, kumaraditya303 and vstinner June 19, 2026 15:36

corona10 mentioned this pull request Jun 19, 2026

gh-151722: Fix data race on frozendict reads during construction in free-threading build #151724

Open

corona10 added needs backport to 3.15 pre-release feature fixes, bugs and security fixes topic-free-threading labels Jun 19, 2026

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gh-151722: Defer GC tracking of frozendict to end of construction#151740

gh-151722: Defer GC tracking of frozendict to end of construction#151740
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