diff --git a/mmtk/src/abi.rs b/mmtk/src/abi.rs
index 3567b5f..e7640b0 100644
--- a/mmtk/src/abi.rs
+++ b/mmtk/src/abi.rs
@@ -342,8 +342,8 @@ pub struct RubyUpcalls {
         data: *mut libc::c_void,
     ),
     pub scan_vm_specific_roots: extern "C" fn(),
-    pub scan_thread_roots: extern "C" fn(),
-    pub scan_thread_root: extern "C" fn(mutator_tls: VMMutatorThread, worker_tls: VMWorkerThread),
+    pub scan_roots_in_mutator_thread:
+        extern "C" fn(mutator_tls: VMMutatorThread, worker_tls: VMWorkerThread),
     pub scan_object_ruby_style: extern "C" fn(object: ObjectReference),
     pub call_gc_mark_children: extern "C" fn(object: ObjectReference),
     pub call_obj_free: extern "C" fn(object: ObjectReference),
diff --git a/mmtk/src/collection.rs b/mmtk/src/collection.rs
index 3cf417a..2b788bb 100644
--- a/mmtk/src/collection.rs
+++ b/mmtk/src/collection.rs
@@ -1,5 +1,6 @@
 use crate::abi::GCThreadTLS;
 
+use crate::api::RubyMutator;
 use crate::{mmtk, upcalls, Ruby};
 use mmtk::memory_manager;
 use mmtk::scheduler::*;
@@ -10,12 +11,16 @@ use std::thread;
 pub struct VMCollection {}
 
 impl Collection<Ruby> for VMCollection {
-    fn stop_all_mutators<F>(tls: VMWorkerThread, _mutator_visitor: F)
+    fn stop_all_mutators<F>(tls: VMWorkerThread, mut mutator_visitor: F)
     where
         F: FnMut(&'static mut mmtk::Mutator<Ruby>),
     {
         (upcalls().stop_the_world)(tls);
         crate::binding().ppp_registry.pin_ppp_children(tls);
+        (upcalls().get_mutators)(
+            Self::notify_mutator_ready::<F>,
+            &mut mutator_visitor as *mut F as *mut _,
+        );
     }
 
     fn resume_mutators(tls: VMWorkerThread) {
@@ -81,3 +86,14 @@ impl Collection<Ruby> for VMCollection {
         (upcalls().vm_live_bytes)()
     }
 }
+
+impl VMCollection {
+    extern "C" fn notify_mutator_ready<F>(mutator_ptr: *mut RubyMutator, data: *mut libc::c_void)
+    where
+        F: FnMut(&'static mut mmtk::Mutator<Ruby>),
+    {
+        let mutator = unsafe { &mut *mutator_ptr };
+        let mutator_visitor = unsafe { &mut *(data as *mut F) };
+        mutator_visitor(mutator);
+    }
+}
diff --git a/mmtk/src/scanning.rs b/mmtk/src/scanning.rs
index 032e33e..53ae710 100644
--- a/mmtk/src/scanning.rs
+++ b/mmtk/src/scanning.rs
@@ -62,7 +62,7 @@ impl Scanning<Ruby> for VMScanning {
     ) {
         let gc_tls = unsafe { GCThreadTLS::from_vwt_check(tls) };
         Self::collect_object_roots_in("scan_thread_root", gc_tls, &mut factory, || {
-            (upcalls().scan_thread_root)(mutator.get_tls(), tls);
+            (upcalls().scan_roots_in_mutator_thread)(mutator.get_tls(), tls);
         });
     }
 
diff --git a/mmtk/src/weak_proc.rs b/mmtk/src/weak_proc.rs
index 270d7bc..f63a58c 100644
--- a/mmtk/src/weak_proc.rs
+++ b/mmtk/src/weak_proc.rs
@@ -148,9 +148,11 @@ trait GlobalTableProcessingWork {
         // `hash_foreach_replace` depends on `gb_object_moved_p` which has to have the semantics
         // of `trace_object` due to the way it is used in `UPDATE_IF_MOVED`.
         let forward_object = |_worker, object: ObjectReference, _pin| {
-            debug_assert!(mmtk::memory_manager::is_mmtk_object(
-                VMObjectModel::ref_to_address(object)
-            ), "{} is not an MMTk object", object);
+            debug_assert!(
+                mmtk::memory_manager::is_mmtk_object(VMObjectModel::ref_to_address(object)),
+                "{} is not an MMTk object",
+                object
+            );
             let result = object.forward();
             trace!("Forwarding reference: {} -> {}", object, result);
             result