gh-150411: fix gc_generation.count race in free-threading (#150413)

Author: LindaSummer

Lib/test/test_free_threading/test_gc.py

@@ -124,6 +124,35 @@ def setter():
         finally:
             gc.set_threshold(*current_threshold)
 
+    def test_get_count(self):
+        class CyclicReference:
+            def __init__(self):
+                self.ref = self
+
+        NUM_ALLOCATORS = 7
+        NUM_READERS = 1
+        NUM_THREADS = NUM_ALLOCATORS + NUM_READERS
+        NUM_ITERS = 1000
+
+        barrier = threading.Barrier(NUM_THREADS)
+
+        def allocator():
+            barrier.wait()
+            for _ in range(NUM_ITERS):
+                CyclicReference()
+
+
+        def reader():
+            barrier.wait()
+            for _ in range(NUM_ITERS):
+                gc.get_count()
+
+        threads = [Thread(target=allocator) for _ in range(NUM_ALLOCATORS)]
+        threads.extend(Thread(target=reader) for _ in range(NUM_READERS))
+
+        with threading_helper.start_threads(threads):
+            pass
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-05-26-00-06-30.gh-issue-150411.u-d-_5.rst

@@ -0,0 +1,2 @@
+Fix a data race in the free-threaded build when :func:`gc.get_count` reads
+the young generation allocation count while another thread updates it.

Modules/gcmodule.c

@@ -233,7 +233,7 @@ gc_get_count_impl(PyObject *module)
                          gcstate->generations[2].count);
 #else
     return Py_BuildValue("(iii)",
-                         gcstate->young.count,
+                         _Py_atomic_load_int_relaxed(&gcstate->young.count),
                          gcstate->old[0].count,
                          gcstate->old[1].count);
 #endif