From 7c3d4248f80c7d4f4538f778354609082aa9d9e7 Mon Sep 17 00:00:00 2001
From: Edward Xu <xuxiangad@gmail.com>
Date: Sun, 7 Jun 2026 01:03:04 +0800
Subject: [PATCH] gh-150411: fix `gc_generation.count` race in free-threading
 (GH-150413) (cherry picked from commit
 12af26d17e4343c1f4ea3a1019bcc34ee76671c3)

Co-authored-by: Edward Xu <xuxiangad@gmail.com>
---
 Lib/test/test_free_threading/test_gc.py       | 29 +++++++++++++++++++
 ...-05-26-00-06-30.gh-issue-150411.u-d-_5.rst |  2 ++
 Modules/gcmodule.c                            |  2 +-
 3 files changed, 32 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Core_and_Builtins/2026-05-26-00-06-30.gh-issue-150411.u-d-_5.rst

diff --git a/Lib/test/test_free_threading/test_gc.py b/Lib/test/test_free_threading/test_gc.py
index cc1888dae48bc03..399010234509408 100644
--- a/Lib/test/test_free_threading/test_gc.py
+++ b/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()
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-05-26-00-06-30.gh-issue-150411.u-d-_5.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-05-26-00-06-30.gh-issue-150411.u-d-_5.rst
new file mode 100644
index 000000000000000..5b19a4fff5ddc73
--- /dev/null
+++ b/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.
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index fe1802c73dab315..7bb1dcc076ee495 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -234,7 +234,7 @@ 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