@@ -287,6 +287,22 @@ def test_index(self):
                     else:
                         self.assertEqual(d.index(element, start, stop), target)
 
+        # Test stop argument
+        for elem in d:
+            index = d.index(elem)
+            self.assertEqual(
+                index,
+                d.index(elem, 0),
+            )
+            self.assertEqual(
+                index,
+                d.index(elem, 0, len(d)),
+            )
+            self.assertEqual(
+                index,
+                d.index(elem, 0, len(d) + 100),
+            )
+
         # Test large start argument
         d = deque(range(0, 10000, 10))
         for step in range(100):
 
@@ -24,6 +24,30 @@ def copy_loop():
 
         threading_helper.run_concurrently([mutate, copy_loop])
 
+    def test_index_race_in_ac(self):
+        # gh-150750: There was a c_default specified as `Py_SIZE(self)`,
+        # it was used without a critical section.
+
+        d = deque(range(100))
+
+        def index():
+            for _ in range(10000):
+                try:
+                    d.index(50)
+                except ValueError:
+                    pass
+
+        def mutate():
+            for _ in range(10000):
+                d.append(0)
+                d.clear()
+                d.extend(range(100))
+                d.appendleft(-1)
+
+        threading_helper.run_concurrently(
+            [index, *[mutate for _ in range(3)]],
+        )
+
 
 if __name__ == "__main__":
     unittest.main()
@@ -0,0 +1 @@
+Fix a race condition in :meth:`collections.deque.index` with free-threading.
@@ -1251,7 +1251,7 @@ _collections.deque.index as deque_index
     deque: dequeobject
     value as v: object
     start: object(converter='_PyEval_SliceIndexNotNone', type='Py_ssize_t', c_default='0') = NULL
-    stop: object(converter='_PyEval_SliceIndexNotNone', type='Py_ssize_t', c_default='Py_SIZE(deque)') = NULL
+    stop: object(converter='_PyEval_SliceIndexNotNone', type='Py_ssize_t', c_default='PY_SSIZE_T_MAX') = NULL
     /
 
 Return first index of value.
@@ -1262,30 +1262,31 @@ Raises ValueError if the value is not present.
 static PyObject *
 deque_index_impl(dequeobject *deque, PyObject *v, Py_ssize_t start,
                  Py_ssize_t stop)
-/*[clinic end generated code: output=df45132753175ef9 input=90f48833a91e1743]*/
+/*[clinic end generated code: output=df45132753175ef9 input=1c3b19632cf3484f]*/
 {
     Py_ssize_t i, n;
     PyObject *item;
     block *b = deque->leftblock;
     Py_ssize_t index = deque->leftindex;
     size_t start_state = deque->state;
     int cmp;
+    Py_ssize_t size = Py_SIZE(deque);
 
     if (start < 0) {
-        start += Py_SIZE(deque);
+        start += size;
         if (start < 0)
             start = 0;
     }
     if (stop < 0) {
-        stop += Py_SIZE(deque);
+        stop += size;
         if (stop < 0)
             stop = 0;
     }
-    if (stop > Py_SIZE(deque))
-        stop = Py_SIZE(deque);
+    if (stop > size)
+        stop = size;
     if (start > stop)
         start = stop;
-    assert(0 <= start && start <= stop && stop <= Py_SIZE(deque));
+    assert(0 <= start && start <= stop && stop <= size);
 
     for (i=0 ; i < start - BLOCKLEN ; i += BLOCKLEN) {
         b = b->rightlink;
 
@@ -3123,7 +3123,7 @@ class path_t_converter(CConverter):
     impl_by_reference = True
     parse_by_reference = True
     default_type = ()
-    c_init_default = "<placeholder>"  # overridden in pre_render(()
+    c_init_default = "<placeholder>"  # overridden in pre_render()
 
     converter = 'path_converter'
 
@@ -3266,7 +3266,7 @@ class confname_converter(CConverter):
         """, argname=argname, converter=self.converter, table=self.table)
 
 [python start generated code]*/
-/*[python end generated code: output=da39a3ee5e6b4b0d input=d58f18bdf3bd3565]*/
+/*[python end generated code: output=da39a3ee5e6b4b0d input=ddbf3ac90a981122]*/
 
 /*[clinic input]
Original file line number	Diff line number	Diff line change
@@ -0,0 +1 @@
	1	+ Fix a race condition in :meth:`collections.deque.index` with free-threading.