gh-148613: Fix race in gc_set_threshold and gc_get_threshold

Lib/test/test_free_threading/test_gc.py

@@ -1,7 +1,7 @@
 import unittest
 
 import threading
-from threading import Thread
+from threading import Barrier, Thread
 from unittest import TestCase
 import gc
 
@@ -94,6 +94,36 @@ def evil():
         thread.start()
         thread.join()
 
+    def test_set_threshold(self):
+        # GH-148613: Setting the GC threshold from another thread could cause a
+        # race between the `gc_should_collect` and `gc_set_threshold` functions.
+        NUM_THREADS = 8
+        NUM_ITERS = 100_000
+        barrier = Barrier(NUM_THREADS)
+
+        class CyclicReference:
+            def __init__(self):
+                self.r = self
+
+        def allocator():
+            barrier.wait()
+            for _ in range(NUM_ITERS):
+                CyclicReference()
+
+        def setter():
+            barrier.wait()
+            for i in range(NUM_ITERS):
+                gc.set_threshold(100 + (i % 100), 10 + (i % 10), 10 + (i % 10))
+
+        current_threshold = gc.get_threshold()
+        try:
+            threads = [Thread(target=allocator) for _ in range(NUM_THREADS - 1)]
+            threads.append(Thread(target=setter))
+            with threading_helper.start_threads(threads):
+                pass
+        finally:
+            gc.set_threshold(*current_threshold)
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-05-24-22-46-49.gh-issue-148613.PLpmyd.rst

@@ -0,0 +1,2 @@
+Fix a data race in the free-threaded build between :func:`gc.set_threshold`
+and garbage collection scheduling during object allocation.

Modules/gcmodule.c

@@ -167,12 +167,12 @@ gc_set_threshold_impl(PyObject *module, int threshold0, int group_right_1,
         gcstate->generations[2].threshold = threshold2;
     }
 #else
-    gcstate->young.threshold = threshold0;
+    _Py_atomic_store_int_relaxed(&gcstate->young.threshold, threshold0);
     if (group_right_1) {
-        gcstate->old[0].threshold = threshold1;
+        _Py_atomic_store_int_relaxed(&gcstate->old[0].threshold, threshold1);
     }
     if (group_right_2) {
-        gcstate->old[1].threshold = threshold2;
+        _Py_atomic_store_int_relaxed(&gcstate->old[1].threshold, threshold2);
     }
 #endif
     Py_RETURN_NONE;
@@ -196,9 +196,9 @@ gc_get_threshold_impl(PyObject *module)
                          gcstate->generations[2].threshold);
 #else
     return Py_BuildValue("(iii)",
-                         gcstate->young.threshold,
-                         gcstate->old[0].threshold,
-                         gcstate->old[1].threshold);
+                         _Py_atomic_load_int_relaxed(&gcstate->young.threshold),
+                         _Py_atomic_load_int_relaxed(&gcstate->old[0].threshold),
+                         _Py_atomic_load_int_relaxed(&gcstate->old[1].threshold));
 #endif
 }
 

Python/gc_free_threading.c

@@ -1996,12 +1996,12 @@ static bool
 gc_should_collect(GCState *gcstate)
 {
     int count = _Py_atomic_load_int_relaxed(&gcstate->young.count);
-    int threshold = gcstate->young.threshold;
+    int threshold = _Py_atomic_load_int_relaxed(&gcstate->young.threshold);
     int gc_enabled = _Py_atomic_load_int_relaxed(&gcstate->enabled);
     if (count <= threshold || threshold == 0 || !gc_enabled) {
         return false;
     }
-    if (gcstate->old[0].threshold == 0) {
+    if (_Py_atomic_load_int_relaxed(&gcstate->old[0].threshold) == 0) {
         // A few tests rely on immediate scheduling of the GC so we ignore the
         // extra conditions if generations[1].threshold is set to zero.
         return true;