diff --git a/gc/mmtk/mmtk.c b/gc/mmtk/mmtk.c
index b8af39c..27a0519 100644
--- a/gc/mmtk/mmtk.c
+++ b/gc/mmtk/mmtk.c
@@ -689,8 +689,7 @@ 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 4cef166..3e2db66 100644
--- a/gc/mmtk/mmtk.h
+++ b/gc/mmtk/mmtk.h
@@ -112,6 +112,8 @@ 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 3515a24..884a40f 100644
--- a/gc/mmtk/src/api.rs
+++ b/gc/mmtk/src/api.rs
@@ -267,6 +267,11 @@ 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::collection::mutators_stopped()
+}
+
 // =============== Object allocation ===============
 
 #[no_mangle]
diff --git a/gc/mmtk/src/collection.rs b/gc/mmtk/src/collection.rs
index 83d046a..667de91 100644
--- a/gc/mmtk/src/collection.rs
+++ b/gc/mmtk/src/collection.rs
@@ -18,6 +18,11 @@ use std::sync::atomic::Ordering;
 use std::thread;
 
 static CURRENT_GC_MAY_MOVE: AtomicBool = AtomicBool::new(false);
+static MUTATORS_STOPPED: AtomicBool = AtomicBool::new(false);
+
+pub fn mutators_stopped() -> bool {
+    MUTATORS_STOPPED.load(Ordering::SeqCst)
+}
 
 pub struct VMCollection {}
 
@@ -31,6 +36,7 @@ impl Collection<Ruby> for VMCollection {
         F: FnMut(&'static mut mmtk::Mutator<Ruby>),
     {
         (upcalls().stop_the_world)();
+        MUTATORS_STOPPED.store(true, Ordering::SeqCst);
 
         if crate::mmtk().get_plan().current_gc_may_move_object() {
             CURRENT_GC_MAY_MOVE.store(true, Ordering::Relaxed);
@@ -48,6 +54,8 @@ impl Collection<Ruby> for VMCollection {
     }
 
     fn resume_mutators(_tls: VMWorkerThread) {
+        MUTATORS_STOPPED.store(false, Ordering::SeqCst);
+
         if CURRENT_GC_MAY_MOVE.load(Ordering::Relaxed) {
             (upcalls().after_updating_jit_code)();
         }