[3.13] pythongh-117657: Enable test_opcache under TSAN (pythonGH-129831)

Fix a few thread-safety bugs to enable test_opcache when run with TSAN:

 * Use relaxed atomics when clearing `ht->_spec_cache.getitem`
   (pythongh-115999)
 * Add temporary suppression for type slot modifications (pythongh-127266)
 * Use atomic load when reading `*dictptr`
(cherry picked from commit f151d27)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Author: colesbury

Lib/test/libregrtest/tsan.py

@@ -13,6 +13,7 @@
     'test_importlib',
     'test_io',
     'test_logging',
+    'test_opcache',
     'test_queue',
     'test_signal',
     'test_socket',

Objects/object.c

@@ -1584,7 +1584,7 @@ _PyObject_GetMethod(PyObject *obj, PyObject *name, PyObject **method)
     else {
         PyObject **dictptr = _PyObject_ComputedDictPointer(obj);
         if (dictptr != NULL) {
-            dict = *dictptr;
+            dict = FT_ATOMIC_LOAD_PTR_ACQUIRE(*dictptr);
         }
         else {
             dict = NULL;

Objects/typeobject.c

@@ -1043,7 +1043,8 @@ type_modified_unlocked(PyTypeObject *type)
     if (PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
         // This field *must* be invalidated if the type is modified (see the
         // comment on struct _specialization_cache):
-        ((PyHeapTypeObject *)type)->_spec_cache.getitem = NULL;
+        FT_ATOMIC_STORE_PTR_RELAXED(
+            ((PyHeapTypeObject *)type)->_spec_cache.getitem, NULL);
     }
 }
 
@@ -1119,7 +1120,8 @@ type_mro_modified(PyTypeObject *type, PyObject *bases) {
     if (PyType_HasFeature(type, Py_TPFLAGS_HEAPTYPE)) {
         // This field *must* be invalidated if the type is modified (see the
         // comment on struct _specialization_cache):
-        ((PyHeapTypeObject *)type)->_spec_cache.getitem = NULL;
+        FT_ATOMIC_STORE_PTR_RELAXED(
+            ((PyHeapTypeObject *)type)->_spec_cache.getitem, NULL);
     }
 }
 

Tools/tsan/suppressions_free_threading.txt

@@ -47,5 +47,8 @@ race_top:PyThreadState_Clear
 # Only seen on macOS, sample: https://gist.github.com/aisk/dda53f5d494a4556c35dde1fce03259c
 race_top:set_default_allocator_unlocked
 
+# gh-127266: type slot updates are not thread-safe (test_opcache.test_load_attr_method_lazy_dict)
+race_top:update_one_slot
+
 # https://gist.github.com/mpage/6962e8870606cfc960e159b407a0cb40
 thread:pthread_create