gh-117657: Make Py_TYPE and Py_SET_TYPE thread safe

Include/internal/pycore_interp.h

@@ -401,7 +401,10 @@ PyAPI_FUNC(PyStatus) _PyInterpreterState_New(
 #define RARE_EVENT_INTERP_INC(interp, name) \
     do { \
         /* saturating add */ \
-        if (interp->rare_events.name < UINT8_MAX) interp->rare_events.name++; \
+        int val = FT_ATOMIC_LOAD_UINT8_RELAXED(interp->rare_events.name); \
+        if (val < UINT8_MAX) { \
+            FT_ATOMIC_STORE_UINT8_RELAXED(interp->rare_events.name, val+1); \
+        } \
         RARE_EVENT_STAT_INC(name); \
     } while (0); \
 

Include/object.h

@@ -246,7 +246,11 @@ _Py_IsOwnedByCurrentThread(PyObject *ob)
 
 // bpo-39573: The Py_SET_TYPE() function must be used to set an object type.
 static inline PyTypeObject* Py_TYPE(PyObject *ob) {
+#ifdef Py_GIL_DISABLED
+    return (PyTypeObject *)_Py_atomic_load_ptr_relaxed(&ob->ob_type);
+#else
     return ob->ob_type;
+#endif
 }
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
 #  define Py_TYPE(ob) Py_TYPE(_PyObject_CAST(ob))
@@ -274,7 +278,11 @@ static inline int Py_IS_TYPE(PyObject *ob, PyTypeObject *type) {
 
 
 static inline void Py_SET_TYPE(PyObject *ob, PyTypeObject *type) {
+#ifdef Py_GIL_DISABLED
+    _Py_atomic_store_ptr(&ob->ob_type, type);
+#else
     ob->ob_type = type;
+#endif
 }
 #if !defined(Py_LIMITED_API) || Py_LIMITED_API+0 < 0x030b0000
 #  define Py_SET_TYPE(ob, type) Py_SET_TYPE(_PyObject_CAST(ob), type)

Lib/test/test_free_threading/test_type.py

@@ -1,3 +1,4 @@
+import threading
 import unittest
 
 from concurrent.futures import ThreadPoolExecutor
@@ -95,6 +96,32 @@ def reader_func():
 
         self.run_one(writer_func, reader_func)
 
+    def test___class___modification(self):
+        class Foo:
+            pass
+
+        class Bar:
+            pass
+
+        thing = Foo()
+        def work():
+            foo = thing
+            for _ in range(10000):
+                foo.__class__ = Bar
+                type(foo)
+                foo.__class__ = Foo
+                type(foo)
+
+
+        threads = []
+        for i in range(NTHREADS):
+            thread = threading.Thread(target=work)
+            thread.start()
+            threads.append(thread)
+
+        for thread in threads:
+            thread.join()
+
     def run_one(self, writer_func, reader_func):
         writer = Thread(target=writer_func)
         readers = []

Tools/tsan/suppressions_free_threading.txt

@@ -39,7 +39,6 @@ race_top:set_contains_key
 # https://gist.github.com/colesbury/d13d033f413b4ad07929d044bed86c35
 race_top:set_discard_entry
 race_top:set_inheritable
-race_top:Py_SET_TYPE
 race_top:_PyDict_CheckConsistency
 race_top:_PyImport_AcquireLock
 race_top:_Py_dict_lookup_threadsafe
@@ -62,7 +61,6 @@ race_top:_PyFrame_Initialize
 race_top:PyInterpreterState_ThreadHead
 race_top:_PyObject_TryGetInstanceAttribute
 race_top:PyThreadState_Next
-race_top:Py_TYPE
 race_top:PyUnstable_InterpreterFrame_GetLine
 race_top:sock_close
 race_top:tstate_delete_common