gh-131141: fix data race in instrumentation while registering callback (#131142)

Author: kumaraditya303

Misc/NEWS.d/next/Core_and_Builtins/2025-03-12-11-19-46.gh-issue-131141.tQz594.rst

@@ -0,0 +1 @@
+Fix data race in :data:`sys.monitoring` instrumentation while registering callback.

Python/instrumentation.c

@@ -1399,9 +1399,10 @@ _PyMonitoring_RegisterCallback(int tool_id, int event_id, PyObject *obj)
     PyInterpreterState *is = _PyInterpreterState_GET();
     assert(0 <= tool_id && tool_id < PY_MONITORING_TOOL_IDS);
     assert(0 <= event_id && event_id < _PY_MONITORING_EVENTS);
-    PyObject *callback = _Py_atomic_exchange_ptr(&is->monitoring_callables[tool_id][event_id],
-                                                 Py_XNewRef(obj));
-
+    _PyEval_StopTheWorld(is);
+    PyObject *callback = is->monitoring_callables[tool_id][event_id];
+    is->monitoring_callables[tool_id][event_id] = Py_XNewRef(obj);
+    _PyEval_StartTheWorld(is);
     return callback;
 }