Data race between compare_generic and insert_combined_dict under free-threading #132641
Description
Bug report
Bug description:
I built main branch and observed the following races under free-threading in cpython 3.14 (Python 3.14.0a7+ experimental free-threading build (heads/main:e42bda94411, Apr 17 2025, 14:08:39) [Clang 18.1.3 (1ubuntu1)])
Race 1
WARNING: ThreadSanitizer: data race (pid=33872)
Read of size 8 at 0x7fffc22d7ad8 by thread T5 (mutexes: read M0):
#0 compare_generic /project/cpython/Objects/dictobject.c:1107:13 (python3.14+0x26edbc) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#1 do_lookup /project/cpython/Objects/dictobject.c:1013:23 (python3.14+0x26edbc)
#2 dictkeys_generic_lookup /project/cpython/Objects/dictobject.c:1132:12 (python3.14+0x26edbc)
#3 _Py_dict_lookup /project/cpython/Objects/dictobject.c:1298:14 (python3.14+0x26edbc)
#4 _PyDict_GetItemRef_KnownHash_LockHeld /project/cpython/Objects/dictobject.c:2330:21 (python3.14+0x272a14) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
...
Previous atomic write of size 8 at 0x7fffc22d7ad8 by thread T6 (mutexes: read M0):
#0 _Py_atomic_store_ptr_release /project/cpython/./Include/cpython/pyatomic_gcc.h:565:3 (python3.14+0x284d66) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#1 insert_combined_dict /project/cpython/Objects/dictobject.c:1748:9 (python3.14+0x284d66)
#2 insertdict /project/cpython/Objects/dictobject.c:1854:13 (python3.14+0x274254) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#3 _PyDict_SetItem_KnownHash_LockHeld /project/cpython/Objects/dictobject.c:2656:12 (python3.14+0x273be9) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#4 _PyDict_SetItem_KnownHash /project/cpython/Objects/dictobject.c:2673:11 (python3.14+0x274635) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
...
Race 2
WARNING: ThreadSanitizer: data race (pid=33872)
Read of size 8 at 0x7fffc22d7ad0 by thread T5 (mutexes: read M0):
#0 compare_generic /project/cpython/Objects/dictobject.c:1110:13 (python3.14+0x26edd5) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#1 do_lookup /project/cpython/Objects/dictobject.c:1013:23 (python3.14+0x26edd5)
#2 dictkeys_generic_lookup /project/cpython/Objects/dictobject.c:1132:12 (python3.14+0x26edd5)
#3 _Py_dict_lookup /project/cpython/Objects/dictobject.c:1298:14 (python3.14+0x26edd5)
#4 _PyDict_GetItemRef_KnownHash_LockHeld /project/cpython/Objects/dictobject.c:2330:21 (python3.14+0x272a14) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
...
Previous atomic write of size 8 at 0x7fffc22d7ad0 by thread T6 (mutexes: read M0):
#0 insert_combined_dict /project/cpython/Objects/dictobject.c (python3.14+0x284d8c) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#1 insertdict /project/cpython/Objects/dictobject.c:1854:13 (python3.14+0x274254) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#2 _PyDict_SetItem_KnownHash_LockHeld /project/cpython/Objects/dictobject.c:2656:12 (python3.14+0x273be9) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
#3 _PyDict_SetItem_KnownHash /project/cpython/Objects/dictobject.c:2673:11 (python3.14+0x274635) (BuildId: d491981d76fd0b67bcb5e01a978d07da019800b8)
...
Full report: https://gist.github.com/vfdev-5/cbb9189043737d023b755191b62951cf
cc @hawkinsp
CPython versions tested on:
CPython main branch
Operating systems tested on:
Linux