Restore no-gil behavior

Include/internal/pycore_object.h

@@ -135,14 +135,39 @@ static inline void _Py_RefcntAdd(PyObject* op, Py_ssize_t n)
         _Py_INCREF_IMMORTAL_STAT_INC();
         return;
     }
+#ifndef Py_GIL_DISABLED
     Py_ssize_t refcnt = _Py_REFCNT(op);
     Py_ssize_t new_refcnt = refcnt + n;
     if (new_refcnt >= (Py_ssize_t)_Py_IMMORTAL_MINIMUM_REFCNT) {
         new_refcnt = _Py_IMMORTAL_INITIAL_REFCNT;
     }
-    Py_SET_REFCNT(op, new_refcnt);
-#ifdef Py_REF_DEBUG
+#  if SIZEOF_VOID_P > 4
+    op->ob_refcnt = (PY_UINT32_T)new_refcnt;
+#  else
+    op->ob_refcnt = new_refcnt;
+#  endif
+#  ifdef Py_REF_DEBUG
     _Py_AddRefTotal(_PyThreadState_GET(), new_refcnt - refcnt);
+#  endif
+#else
+    if (_Py_IsOwnedByCurrentThread(op)) {
+        uint32_t local = op->ob_ref_local;
+        Py_ssize_t refcnt = (Py_ssize_t)local + n;
+#  if PY_SSIZE_T_MAX > UINT32_MAX
+        if (refcnt > (Py_ssize_t)UINT32_MAX) {
+            // Make the object immortal if the 32-bit local reference count
+            // would overflow.
+            refcnt = _Py_IMMORTAL_REFCNT_LOCAL;
+        }
+#  endif
+        _Py_atomic_store_uint32_relaxed(&op->ob_ref_local, (uint32_t)refcnt);
+    }
+    else {
+        _Py_atomic_add_ssize(&op->ob_ref_shared, (n << _Py_REF_SHARED_SHIFT));
+    }
+#  ifdef Py_REF_DEBUG
+    _Py_AddRefTotal(_PyThreadState_GET(), n);
+#  endif
 #endif
     // Although the ref count was increased by `n` (which may be greater than 1)
     // it is only a single increment (i.e. addition) operation, so only 1 refcnt

Include/refcount.h

@@ -158,6 +158,9 @@ static inline void Py_SET_REFCNT(PyObject *ob, Py_ssize_t refcnt) {
         return;
     }
 #ifndef Py_GIL_DISABLED
+    if (refcnt >= (Py_ssize_t)_Py_IMMORTAL_MINIMUM_REFCNT) {
+        refcnt = _Py_IMMORTAL_INITIAL_REFCNT;
+    }
 #if SIZEOF_VOID_P > 4
     ob->ob_refcnt = (PY_UINT32_T)refcnt;
 #else