diff --git a/gc/gc.h b/gc/gc.h
index 6f8b829..df70942 100644
--- a/gc/gc.h
+++ b/gc/gc.h
@@ -31,6 +31,7 @@ enum rb_gc_vm_weak_tables {
     RB_GC_VM_ID2REF_TABLE,
     RB_GC_VM_GENERIC_FIELDS_TABLE,
     RB_GC_VM_FROZEN_STRINGS_TABLE,
+    RB_GC_VM_CC_REFINEMENT_TABLE,
     RB_GC_VM_WEAK_TABLE_COUNT
 };
 
diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index 41e7f13..e00be5d 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -515,8 +515,7 @@ rb_gc_impl_start(void *objspace_ptr, bool full_mark, bool immediate_mark, bool i
 bool
 rb_gc_impl_during_gc_p(void *objspace_ptr)
 {
-    // TODO
-    return false;
+    return mmtk_gc_in_progress_p();
 }
 
 static void
diff --git a/gc/mmtk/mmtk.h b/gc/mmtk/mmtk.h
index 72b4d9d..8a4f65e 100644
--- a/gc/mmtk/mmtk.h
+++ b/gc/mmtk/mmtk.h
@@ -99,6 +99,8 @@ void mmtk_set_gc_enabled(bool enable);
 
 bool mmtk_gc_enabled_p(void);
 
+bool mmtk_gc_in_progress_p(void);
+
 MMTk_Address mmtk_alloc(MMTk_Mutator *mutator,
                         size_t size,
                         size_t align,
diff --git a/gc/mmtk/src/api.rs b/gc/mmtk/src/api.rs
index a1b94d5..454025b 100644
--- a/gc/mmtk/src/api.rs
+++ b/gc/mmtk/src/api.rs
@@ -207,6 +207,11 @@ pub extern "C" fn mmtk_gc_enabled_p() -> bool {
     crate::CONFIGURATION.gc_enabled.load(Ordering::Relaxed)
 }
 
+#[no_mangle]
+pub extern "C" fn mmtk_gc_in_progress_p() -> bool {
+    crate::mmtk().gc_in_progress()
+}
+
 // =============== Object allocation ===============
 
 #[no_mangle]