gh-148613: Fix race in gc_set_threshold and gc_get_threshold

Include/internal/pycore_interp_structs.h

@@ -229,6 +229,7 @@ struct _gc_runtime_state {
 #ifndef Py_GIL_DISABLED
     struct gc_generation generations[NUM_GENERATIONS];
 #else
+    PyMutex generations_mutex; // Mutex to guard threshold access to the generations below.
     struct gc_generation young;
     struct gc_generation old[2];
 #endif

Lib/test/test_free_threading/test_gc.py

@@ -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 = threading.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

@@ -167,13 +167,15 @@ gc_set_threshold_impl(PyObject *module, int threshold0, int group_right_1,
         gcstate->generations[2].threshold = threshold2;
     }
 #else
+    PyMutex_Lock(&gcstate->generations_mutex);
     gcstate->young.threshold = threshold0;
     if (group_right_1) {
         gcstate->old[0].threshold = threshold1;
     }
     if (group_right_2) {
         gcstate->old[1].threshold = threshold2;
     }
+    PyMutex_Unlock(&gcstate->generations_mutex);
 #endif
     Py_RETURN_NONE;
 }
@@ -195,10 +197,15 @@ gc_get_threshold_impl(PyObject *module)
                          gcstate->generations[1].threshold,
                          gcstate->generations[2].threshold);
 #else
+    PyMutex_Lock(&gcstate->generations_mutex);
+    int young_threshold = gcstate->young.threshold;
+    int old_0_threshold = gcstate->old[0].threshold;
+    int old_1_threshold = gcstate->old[1].threshold;
+    PyMutex_Unlock(&gcstate->generations_mutex);
     return Py_BuildValue("(iii)",
-                         gcstate->young.threshold,
-                         gcstate->old[0].threshold,
-                         gcstate->old[1].threshold);
+                         young_threshold,
+                         old_0_threshold,
+                         old_1_threshold);
 #endif
 }
 

Python/gc_free_threading.c

@@ -1995,13 +1995,22 @@ cleanup_worklist(struct worklist *worklist)
 static bool
 gc_should_collect(GCState *gcstate)
 {
+    int gc_enabled = _Py_atomic_load_int_relaxed(&gcstate->enabled);
+    if (!gc_enabled) {
+        return false;
+    }
+
     int count = _Py_atomic_load_int_relaxed(&gcstate->young.count);
+
+    PyMutex_Lock(&gcstate->generations_mutex);
     int threshold = gcstate->young.threshold;
-    int gc_enabled = _Py_atomic_load_int_relaxed(&gcstate->enabled);
-    if (count <= threshold || threshold == 0 || !gc_enabled) {
+    int old_0_threshold = gcstate->old[0].threshold;
+    PyMutex_Unlock(&gcstate->generations_mutex);
+
+    if (count <= threshold || threshold == 0) {
         return false;
     }
-    if (gcstate->old[0].threshold == 0) {
+    if (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;