@@ -834,7 +834,7 @@ static PyMethodDef setiter_methods[] = {
 
 static PyObject *setiter_iternext(setiterobject *si)
 {
-    PyObject *key;
+    PyObject *key = NULL;
     Py_ssize_t i, mask;
     setentry *entry;
     PySetObject *so = si->si_set;
@@ -843,29 +843,38 @@ static PyObject *setiter_iternext(setiterobject *si)
         return NULL;
     assert (PyAnySet_Check(so));
 
-    if (si->si_used != so->used) {
+    Py_ssize_t so_used = FT_ATOMIC_LOAD_SSIZE(so->used);
+    Py_ssize_t si_used = FT_ATOMIC_LOAD_SSIZE(si->si_used);
+    if (si_used != so_used) {
         PyErr_SetString(PyExc_RuntimeError,
                         "Set changed size during iteration");
         si->si_used = -1; /* Make this state sticky */
         return NULL;
     }
 
+    Py_BEGIN_CRITICAL_SECTION(so);
     i = si->si_pos;
     assert(i>=0);
     entry = so->table;
     mask = so->mask;
-    while (i <= mask && (entry[i].key == NULL || entry[i].key == dummy))
+    while (i <= mask && (entry[i].key == NULL || entry[i].key == dummy)) {
         i++;
+    }
+    if (i <= mask) {
+        key = Py_NewRef(entry[i].key);
+    }
+    Py_END_CRITICAL_SECTION();
     si->si_pos = i+1;
-    if (i > mask)
+    if (key == NULL) {
         goto fail;
+    }
     si->len--;
-    key = entry[i].key;
-    return Py_NewRef(key);
+    return key;
 
 fail:
     si->si_set = NULL;
     Py_DECREF(so);
+    Py_XDECREF(key);
     return NULL;
 }