python · nascheme · Jul 3, 2026 · Jun 20, 2026 · Jun 20, 2026
diff --git a/Lib/test/test_free_threading/test_sys.py b/Lib/test/test_free_threading/test_sys.py
@@ -4,6 +4,27 @@
 
 
 class SysModuleTest(unittest.TestCase):
+    @unittest.skipUnless(hasattr(sys, "setdlopenflags"),
+                         "test needs sys.setdlopenflags()")
+    def test_dlopenflags_concurrent(self):
+        # gh-151644: getdlopenflags() and setdlopenflags() must be safe to
+        # call concurrently in free-threaded builds.
+        original = sys.getdlopenflags()
+        self.addCleanup(sys.setdlopenflags, original)
+
+        # Use a small set of known-valid flag values to avoid integer overflow.
+        flag_values = [1, 2, 256, 257]
+
+        def worker(worker_id):
+            for i in range(20_000):
+                if worker_id % 2 == 0:
+                    sys.getdlopenflags()
+                else:
+                    sys.setdlopenflags(flag_values[worker_id % len(flag_values)])
+
+        workers = [lambda i=i: worker(i) for i in range(6)]
+        threading_helper.run_concurrently(workers)
+
     def test_int_max_str_digits_thread(self):
         # gh-151218: Check that it's safe to call get_int_max_str_digits() and
         # set_int_max_str_digits() in parallel. Previously, this test triggered

diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-18-00-00-00.gh-issue-151644.5cFffN.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-18-00-00-00.gh-issue-151644.5cFffN.rst
@@ -0,0 +1,4 @@
+Fix a data race in :func:`sys.setdlopenflags` and :func:`sys.getdlopenflags`
+when called concurrently in the free-threaded build. The underlying
+``_PyImport_GetDLOpenFlags`` and ``_PyImport_SetDLOpenFlags`` functions now
+use atomic load/store operations.
@@ -908,13 +908,13 @@ _PyImport_SwapPackageContext(const char *newcontext)
 int
 _PyImport_GetDLOpenFlags(PyInterpreterState *interp)
 {
-    return DLOPENFLAGS(interp);
+    return FT_ATOMIC_LOAD_INT_RELAXED(DLOPENFLAGS(interp));
 }
 
 void
 _PyImport_SetDLOpenFlags(PyInterpreterState *interp, int new_val)
 {
-    DLOPENFLAGS(interp) = new_val;
+    FT_ATOMIC_STORE_INT_RELAXED(DLOPENFLAGS(interp), new_val);
 }
 #endif  // HAVE_DLOPEN