gh-127266: avoid data races when updating type slots (gh-131174)

In the free-threaded build, avoid data races caused by updating type slots
or type flags after the type was initially created.  For those (typically
rare) cases, use the stop-the-world mechanism.  Remove the use of atomics
when reading or writing type flags.  The use of atomics is not sufficient to
avoid races (since flags are sometimes read without a lock and without
atomics) and are no longer required.

1 files changed, 14 insertions, 0 deletions

diff --git a/Python/ceval.c b/Python/ceval.c
index fb72fd4..b7b7f9c 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -138,6 +138,19 @@
 #endif
 
 
+static void
+check_invalid_reentrancy(void)
+{
+#if defined(Py_DEBUG) && defined(Py_GIL_DISABLED)
+    // In the free-threaded build, the interpreter must not be re-entered if
+    // the world-is-stopped.  If so, that's a bug somewhere (quite likely in
+    // the painfully complex typeobject code).
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    assert(!interp->stoptheworld.world_stopped);
+#endif
+}
+
+
 #ifdef Py_DEBUG
 static void
 dump_item(_PyStackRef item)
@@ -995,6 +1008,7 @@ PyObject* _Py_HOT_FUNCTION DONT_SLP_VECTORIZE
 _PyEval_EvalFrameDefault(PyThreadState *tstate, _PyInterpreterFrame *frame, int throwflag)
 {
     _Py_EnsureTstateNotNULL(tstate);
+    check_invalid_reentrancy();
     CALL_STAT_INC(pyeval_calls);
 
 #if USE_COMPUTED_GOTOS && !Py_TAIL_CALL_INTERP