fix GC event synchronization

(1) gc_verify_internal_consistency() use barrier locking
    for consistency while `during_gc == true` at the end
    of the sweep on `RGENGC_CHECK_MODE >= 2`.
(2) `rb_objspace_reachable_objects_from()` is called without
    VM synchronization and it checks `during_gc != true`.

So (1) and (2) causes BUG because of `during_gc == true`.
To prevent this error, wait for VM barrier on `during_gc == false`
and introduce VM locking on `rb_objspace_reachable_objects_from()`.

http://ci.rvm.jp/results/trunk-asserts@phosphorus-docker/3830088

gc.c

@@ -5722,10 +5722,6 @@ gc_sweep_finish(rb_objspace_t *objspace)
 
     gc_event_hook(objspace, RUBY_INTERNAL_EVENT_GC_END_SWEEP, 0);
     gc_mode_transition(objspace, gc_mode_none);
-
-#if RGENGC_CHECK_MODE >= 2
-    gc_verify_internal_consistency(objspace);
-#endif
 }
 
 static int
@@ -9340,6 +9336,14 @@ gc_exit(rb_objspace_t *objspace, enum gc_enter_event event, unsigned int *lock_l
     mjit_gc_exit_hook();
     gc_exit_clock(objspace, event);
     RB_VM_LOCK_LEAVE_LEV(lock_lev);
+
+#if RGENGC_CHECK_MODE >= 2
+    if (event == gc_enter_event_sweep_continue && gc_mode(objspace) == gc_mode_none) {
+        GC_ASSERT(!during_gc);
+        // sweep finished
+        gc_verify_internal_consistency(objspace);
+    }
+#endif
 }
 
 static void *
@@ -11226,6 +11230,8 @@ rb_objspace_reachable_objects_from(VALUE obj, void (func)(VALUE, void *), void *
 {
     rb_objspace_t *objspace = &rb_objspace;
 
+    RB_VM_LOCK_ENTER();
+    {
         if (during_gc) rb_bug("rb_objspace_reachable_objects_from() is not supported while during_gc == true");
 
         if (is_markable_object(objspace, obj)) {
@@ -11240,6 +11246,8 @@ rb_objspace_reachable_objects_from(VALUE obj, void (func)(VALUE, void *), void *
             cr->mfd = prev_mfd;
         }
     }
+    RB_VM_LOCK_LEAVE();
+}
 
 struct root_objects_data {
     const char *category;