pythongh-129533: Update PyGC_Enable/Disable/IsEnabled to use CAS operation

Author: corona10

Misc/NEWS.d/next/C_API/2025-02-02-12-58-21.gh-issue-129533.dFfqkT.rst

@@ -0,0 +1,3 @@
+Update :c:func:`PyGC_Enable()`, :c:func:`PyGC_Disable()`,
+:c:func:`PyGC_IsEnabled()` to use CAS operation for thread-safety at
+free-threading build. Patch by Donghee Na.

Python/gc_free_threading.c

@@ -1952,25 +1952,29 @@ int
 PyGC_Enable(void)
 {
     GCState *gcstate = get_gc_state();
-    int old_state = gcstate->enabled;
-    gcstate->enabled = 1;
+    int old_state;
+    do {
+        old_state = gcstate->enabled;
+    } while (!_Py_atomic_compare_exchange_int(&gcstate->enabled, &old_state, 1));
     return old_state;
 }
 
 int
 PyGC_Disable(void)
 {
     GCState *gcstate = get_gc_state();
-    int old_state = gcstate->enabled;
-    gcstate->enabled = 0;
+    int old_state;
+    do {
+        old_state = gcstate->enabled;
+    } while (!_Py_atomic_compare_exchange_int(&gcstate->enabled, &old_state, 0));
     return old_state;
 }
 
 int
 PyGC_IsEnabled(void)
 {
     GCState *gcstate = get_gc_state();
-    return gcstate->enabled;
+    return _Py_atomic_load_int(&(gcstate->enabled));
 }
 
 /* Public API to invoke gc.collect() from C */