gh-129732: Fix race on shared->array in qsbr code under free-threading (gh-129738)

hawkinsp · web-flow · commit b4ff8b22b306 · 2025-02-06T18:49:29.000Z
The read of `shared-&gt;array` should happen under the lock to avoid a race.
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-02-06-17-57-33.gh-issue-129732.yl97oq.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-02-06-17-57-33.gh-issue-129732.yl97oq.rst
@@ -0,0 +1 @@
+Fixed a race in ``_Py_qsbr_reserve`` in the free threading build.
diff --git a/Python/qsbr.c b/Python/qsbr.c
@@ -205,15 +205,15 @@ _Py_qsbr_reserve(PyInterpreterState *interp)
         }
         _PyEval_StartTheWorld(interp);
     }
-    PyMutex_Unlock(&shared->mutex);
-
-    if (qsbr == NULL) {
-        return -1;
-    }
 
     // Return an index rather than the pointer because the array may be
     // resized and the pointer invalidated.
-    return (struct _qsbr_pad *)qsbr - shared->array;
+    Py_ssize_t index = -1;
+    if (qsbr != NULL) {
+        index = (struct _qsbr_pad *)qsbr - shared->array;
+    }
+    PyMutex_Unlock(&shared->mutex);
+    return index;
 }
 
 void

Original file line number	Diff line number	Diff line change
`@@ -0,0 +1 @@`
	`1`	+Fixed a race in ``_Py_qsbr_reserve`` in the free threading build.