[3.13] gh-128717: Stop-the-world when setting the recursion limit (GH-128741)

Lib/test/test_sys.py

@@ -362,6 +362,36 @@ def test_setrecursionlimit_to_depth(self):
         finally:
             sys.setrecursionlimit(old_limit)
 
+    @unittest.skipUnless(support.Py_GIL_DISABLED, "only meaningful if the GIL is disabled")
+    @threading_helper.requires_working_threading()
+    def test_racing_recursion_limit(self):
+        from threading import Thread
+        def something_recursive():
+            def count(n):
+                if n > 0:
+                    return count(n - 1) + 1
+                return 0
+
+            count(50)
+
+        def set_recursion_limit():
+            for limit in range(100, 200):
+                sys.setrecursionlimit(limit)
+
+        threads = []
+        for _ in range(5):
+            threads.append(Thread(target=set_recursion_limit))
+
+        for _ in range(5):
+            threads.append(Thread(target=something_recursive))
+
+        with threading_helper.catch_threading_exception() as cm:
+            with threading_helper.start_threads(threads):
+                pass
+
+            if cm.exc_value:
+                raise cm.exc_value
+
     def test_getwindowsversion(self):
         # Raise SkipTest if sys doesn't have getwindowsversion attribute
         test.support.get_attribute(sys, "getwindowsversion")

Misc/NEWS.d/next/Core_and_Builtins/2025-01-11-12-39-17.gh-issue-128717.i65d06.rst

@@ -0,0 +1,2 @@
+Fix a crash when setting the recursion limit while other threads are active
+on the :term:`free threaded <free threading>` build.

Python/ceval.c

@@ -265,12 +265,16 @@ void
 Py_SetRecursionLimit(int new_limit)
 {
     PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyEval_StopTheWorld(interp);
+    HEAD_LOCK(interp->runtime);
     interp->ceval.recursion_limit = new_limit;
     for (PyThreadState *p = interp->threads.head; p != NULL; p = p->next) {
         int depth = p->py_recursion_limit - p->py_recursion_remaining;
         p->py_recursion_limit = new_limit;
         p->py_recursion_remaining = new_limit - depth;
     }
+    HEAD_UNLOCK(interp->runtime);
+    _PyEval_StartTheWorld(interp);
 }
 
 /* The function _Py_EnterRecursiveCallTstate() only calls _Py_CheckRecursiveCall()