gh-133467: Fix typeobject tp_base race in free threading

[LindaSummer]

Issue

#133467

Proposed Changes

• Fix the race of PyTypeObject::tp_base between typemember __base__ and set of __bases__.
• Add test case in unit test.

Comment

Root cause

Since the __bases__'s getter and setter are protetcted by TYPE_LOCK.

cpython/Objects/typeobject.c

Line 1916 in 289360a

ASSERT_TYPE_LOCK_HELD();

cpython/Objects/typeobject.c

Lines 586 to 596 in 289360a

	_PyType_GetBases(PyTypeObject*self)
	{
	PyObject*res;
	BEGIN_TYPE_LOCK();
	res=lookup_tp_bases(self);
	Py_INCREF(res);
	END_TYPE_LOCK();
	returnres;
	}

So it is safe for concurrency.

But the update of bases will re-evaluate the tp_base again and would be in race with the direct access of __base__.

cpython/Objects/typeobject.c

Line 1459 in 289360a

{"__base__", _Py_T_OBJECT, offsetof(PyTypeObject, tp_base), Py_READONLY},

How to fix

At the beginning I try to use the TYPE_LOCK to protect the tb_base for __base__ access.

But it's a directly memory access by offset and I have no more idea except StopTheWorld.

So I add a STW for the updating of tp_base in setter.

Please let me know if we have a better way for it. 😊

TSAN output for this race

Here is the TSAN core output of this race.

WARNING: ThreadSanitizer: data race (pid=1754505)
Write of size 8 at 0x7fb1ed269910 by thread T27:
#0 type_set_bases_unlocked /home/someuser/project/cpython/Objects/typeobject.c:1924:19 (python+0x6b265a) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#1 type_set_bases /home/someuser/project/cpython/Objects/typeobject.c:2009:15 (python+0x6b265a)
#2 getset_set /home/someuser/project/cpython/Objects/descrobject.c:250:16 (python+0x5a8fe6) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#3 type_setattro /home/someuser/project/cpython/Objects/typeobject.c:6532:19 (python+0x6a2ac5) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#4 PyObject_SetAttr /home/someuser/project/cpython/Objects/object.c:1476:15 (python+0x649360) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#5 _PyEval_EvalFrameDefault /home/someuser/project/cpython/Python/generated_cases.c.h:10750:27 (python+0x7bd953) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#6 _PyEval_EvalFrame /home/someuser/project/cpython/./Include/internal/pycore_ceval.h:121:16 (python+0x79c3a0) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#7 _PyEval_Vector /home/someuser/project/cpython/Python/ceval.c:2005:12 (python+0x79c3a0)
#8 _PyFunction_Vectorcall /home/someuser/project/cpython/Objects/call.c (python+0x590def) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#9 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x5956b6) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#10 method_vectorcall /home/someuser/project/cpython/Objects/classobject.c:73:20 (python+0x5956b6)
#11 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x7f4a21) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#12 context_run /home/someuser/project/cpython/Python/context.c:728:29 (python+0x7f4a21)
#13 _PyEval_EvalFrameDefault /home/someuser/project/cpython/Python/generated_cases.c.h:3710:35 (python+0x7a9401) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#14 _PyEval_EvalFrame /home/someuser/project/cpython/./Include/internal/pycore_ceval.h:121:16 (python+0x79c3a0) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#15 _PyEval_Vector /home/someuser/project/cpython/Python/ceval.c:2005:12 (python+0x79c3a0)
#16 _PyFunction_Vectorcall /home/someuser/project/cpython/Objects/call.c (python+0x590def) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#17 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x5956b6) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#18 method_vectorcall /home/someuser/project/cpython/Objects/classobject.c:73:20 (python+0x5956b6)
#19 _PyVectorcall_Call /home/someuser/project/cpython/Objects/call.c:273:16 (python+0x590a87) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#20 _PyObject_Call /home/someuser/project/cpython/Objects/call.c:348:16 (python+0x590a87)
#21 PyObject_Call /home/someuser/project/cpython/Objects/call.c:373:12 (python+0x590af5) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#22 thread_run /home/someuser/project/cpython/./Modules/_threadmodule.c:387:21 (python+0x96c152) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#23 pythread_wrapper /home/someuser/project/cpython/Python/thread_pthread.h:234:5 (python+0x8a0317) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
Previous atomic read of size 8 at 0x7fb1ed269910 by thread T22:
#0 _Py_atomic_load_ptr /home/someuser/project/cpython/./Include/cpython/pyatomic_gcc.h:300:18 (python+0x886ef1) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#1 _Py_TryIncrefCompare /home/someuser/project/cpython/./Include/internal/pycore_object.h:603:15 (python+0x886ef1)
#2 PyMember_GetOne /home/someuser/project/cpython/Python/structmember.c:91:18 (python+0x8869ca) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#3 member_get /home/someuser/project/cpython/Objects/descrobject.c:180:12 (python+0x5a8a06) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#4 _Py_type_getattro_impl /home/someuser/project/cpython/Objects/typeobject.c:6363:19 (python+0x6a0992) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#5 _Py_type_getattro /home/someuser/project/cpython/Objects/typeobject.c:6424:12 (python+0x6a0ca0) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#6 PyObject_GetAttr /home/someuser/project/cpython/Objects/object.c:1313:18 (python+0x648a3e) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#7 _PyObject_GetMethodStackRef /home/someuser/project/cpython/Objects/object.c:1706:25 (python+0x64b746) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#8 _PyEval_EvalFrameDefault /home/someuser/project/cpython/Python/generated_cases.c.h:7844:35 (python+0x7b55b4) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#9 _PyEval_EvalFrame /home/someuser/project/cpython/./Include/internal/pycore_ceval.h:121:16 (python+0x79c3a0) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#10 _PyEval_Vector /home/someuser/project/cpython/Python/ceval.c:2005:12 (python+0x79c3a0)
#11 _PyFunction_Vectorcall /home/someuser/project/cpython/Objects/call.c (python+0x590def) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#12 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x5956b6) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#13 method_vectorcall /home/someuser/project/cpython/Objects/classobject.c:73:20 (python+0x5956b6)
#14 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x7f4a21) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#15 context_run /home/someuser/project/cpython/Python/context.c:728:29 (python+0x7f4a21)
#16 _PyEval_EvalFrameDefault /home/someuser/project/cpython/Python/generated_cases.c.h:3710:35 (python+0x7a9401) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#17 _PyEval_EvalFrame /home/someuser/project/cpython/./Include/internal/pycore_ceval.h:121:16 (python+0x79c3a0) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#18 _PyEval_Vector /home/someuser/project/cpython/Python/ceval.c:2005:12 (python+0x79c3a0)
#19 _PyFunction_Vectorcall /home/someuser/project/cpython/Objects/call.c (python+0x590def) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#20 _PyObject_VectorcallTstate /home/someuser/project/cpython/./Include/internal/pycore_call.h:169:11 (python+0x5956b6) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#21 method_vectorcall /home/someuser/project/cpython/Objects/classobject.c:73:20 (python+0x5956b6)
#22 _PyVectorcall_Call /home/someuser/project/cpython/Objects/call.c:273:16 (python+0x590a87) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#23 _PyObject_Call /home/someuser/project/cpython/Objects/call.c:348:16 (python+0x590a87)
#24 PyObject_Call /home/someuser/project/cpython/Objects/call.c:373:12 (python+0x590af5) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#25 thread_run /home/someuser/project/cpython/./Modules/_threadmodule.c:387:21 (python+0x96c152) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)
#26 pythread_wrapper /home/someuser/project/cpython/Python/thread_pthread.h:234:5 (python+0x8a0317) (BuildId: 111d1b338abaedccddf0af54b80c4dc6c30a747f)

• Issue: Data races in typeobject.c for type structure updates #133467

[LindaSummer]

Hi @nascheme,

Thansk very much for your guidance. 😊

I have created a case for the race of tb_base.

Please help take a look.

Best Regards,
Edward

[LindaSummer]

Hi @nascheme and @colesbury ,

Sorry to bother you. 😊
Could we take a review on this PR?

Hope to get suggestions from you!

Best Regards,
Edward

[nascheme]

LGTM. Using types_stop_world() and types_start_world() would be a little cleaner, IMHO but that's minor.

[colesbury]

I think we might need the helper that avoids releasing the critical section here. I think it might not be safe otherwise

…

On Wed, Oct 29, 2025 at 1:01 PM Neil Schemenauer ***@***.***> wrote: *nascheme* left a comment (python/cpython#140549) <#140549 (comment)> LGTM. Using types_stop_world() and types_start_world() would be a little cleaner, IMHO but that's minor. — Reply to this email directly, view it on GitHub <#140549 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAFAD6VCRC3T5PAJC5UUTB332DXGJAVCNFSM6AAAAACKDZLQCOVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMZTINRSG4YDAMZQG4> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[nascheme]

I think we might need the helper that avoids releasing the critical section
here.

We have this code in apply_type_slot_updates(), which I think could be used here too:

 type_lock_prevent_release();
types_stop_world();
apply_slot_updates(updates);
types_start_world();
type_lock_allow_release();

[LindaSummer]

Hi @nascheme , @colesbury and @kumaraditya303 ,

I have updated the PR as suggested in the review comment. 😄

Please help take a review.

Wish you a good day!

Best Regards,
Edward

[colesbury]

You'll need to update the calls to use type_lock_prevent_release() / type_lock_allow_release() because we do not want to release the type lock when stopping the world. See the pattern and comment in apply_type_slot_updates.

Also, please remove the associated suppression in Tools/tsan/suppressions_free_threading.txt

[LindaSummer]

You'll need to update the calls to use type_lock_prevent_release() / type_lock_allow_release() because we do not want to release the type lock when stopping the world. See the pattern and comment in apply_type_slot_updates.

Also, please remove the associated suppression in Tools/tsan/suppressions_free_threading.txt

Hi @colesbury ,

Thanks very much for your help and suggestion! 😊

I have added the type_lock_prevent_release() / type_lock_allow_release() to guard the STW.

Please correct me if I misunderstand.

Best Regards,
Edward

[LindaSummer]

Hi @nascheme , @colesbury and @kumaraditya303 ,

Thanks very much for your patience and kind suggestions! 😊
I have updated PR as commented.
Please help take a review.

Wish you a good day!

Best Regards,
Edward

[colesbury]

LGTM, thanks!

[miss-islington-app]

Thanks @LindaSummer for the PR, and @colesbury for merging it 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

[miss-islington-app]

Sorry, @LindaSummer and @colesbury, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker b83f379a972c001864d3593cd64fc07e7c7f375f 3.14

[colesbury]

Backport probably depends on backporting:

• gh-127266: avoid data races when updating type slots v2 #133177
• gh-140126: Fix compile error if --with-assertions is enabled. #140133

[nascheme]

Yes, this needs gh-133177 to be backported too and that's a pretty complicated change. So, it feels a bit risky to backport. My guess is that these races would be quite rare in real Python code. It's rare that Python code mutates types in ways that trigger these races. Maybe wait a bit more?

[serhiy-storchaka]

Please don't forget about backporting or remove the "needs backport to 3.14" label if backporting is not planned.