Make RCLASS_EXT(c)->subclasses a doubly linked list

Updating RCLASS_PARENT_SUBCLASSES and RCLASS_MODULE_SUBCLASSES while
compacting can trigger the read barrier. This commit makes
RCLASS_SUBCLASSES a doubly linked list with a dedicated head object so
that we can add and remove entries from the list without having to touch
an object in the Ruby heap

1 files changed, 2 insertions, 13 deletions

diff --git a/gc.c b/gc.c
index 60387d2ab3..b36688979e 100644
--- a/gc.c
+++ b/gc.c
@@ -3134,15 +3134,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
             rb_id_table_foreach_values(RCLASS_CVC_TBL(obj), cvar_table_free_i, NULL);
             rb_id_table_free(RCLASS_CVC_TBL(obj));
 	}
-	if (RCLASS_SUBCLASSES(obj)) {
-	    if (BUILTIN_TYPE(obj) == T_MODULE) {
-		rb_class_detach_module_subclasses(obj);
-	    }
-	    else {
-		rb_class_detach_subclasses(obj);
-	    }
-	    RCLASS_SUBCLASSES(obj) = NULL;
-	}
+        rb_class_remove_subclass_head(obj);
 	rb_class_remove_from_module_subclasses(obj);
 	rb_class_remove_from_super_subclasses(obj);
 #if !USE_RVARGC
@@ -3307,10 +3299,7 @@ obj_free(rb_objspace_t *objspace, VALUE obj)
 	if (RCLASS_CALLABLE_M_TBL(obj) != NULL) {
 	    rb_id_table_free(RCLASS_CALLABLE_M_TBL(obj));
 	}
-	if (RCLASS_SUBCLASSES(obj)) {
-	    rb_class_detach_subclasses(obj);
-	    RCLASS_SUBCLASSES(obj) = NULL;
-	}
+        rb_class_remove_subclass_head(obj);
         cc_table_free(objspace, obj, FALSE);
 	rb_class_remove_from_module_subclasses(obj);
 	rb_class_remove_from_super_subclasses(obj);