gh-117657: Fix race data race in _Py_IsOwnedByCurrentThread()

[mpage]

We need to load ob_tid atomically; it may be set to zero in another thread without any intervening operations that establish a happens-before relationship.

Using a relaxed load should be sufficient; we do not depend on the store publishing any other information.

Examples:

The load in _Py_IsOwnedByCurrentThread():

cpython/Include/object.h

Line 306 in 4b10e20

returnob->ob_tid==_Py_ThreadId();

races with the store in _Py_ExplicitMergeRefcount() (example):

cpython/Objects/object.c

Line 421 in 4b10e20

_Py_atomic_store_uintptr_relaxed(&op->ob_tid, 0);

and the store in _Py_MergeZeroLocalRefcount() (example):

cpython/Objects/object.c

Line 378 in 4b10e20

_Py_atomic_store_uintptr_relaxed(&op->ob_tid, 0);

• Issue: Make TSAN tests pass with the GIL disabled in free-threaded builds #117657