diff --git a/.github/scripts/ci-test-extended.sh b/.github/scripts/ci-test-extended.sh
index fbbed459..4d585d78 100755
--- a/.github/scripts/ci-test-extended.sh
+++ b/.github/scripts/ci-test-extended.sh
@@ -19,6 +19,7 @@ $cur/ci-build.sh
 # Test
 $cur/ci-test-only-normal.sh
 $cur/ci-test-only-normal-no-compressed-oops.sh
+$cur/ci-test-only-weak-ref.sh
 
 # Build with extreme assertions
 MMTK_EXTREME_ASSERTIONS=1 $cur/ci-build.sh
diff --git a/.github/scripts/ci-test-only-weak-ref.sh b/.github/scripts/ci-test-only-weak-ref.sh
new file mode 100755
index 00000000..15bc2f76
--- /dev/null
+++ b/.github/scripts/ci-test-only-weak-ref.sh
@@ -0,0 +1,5 @@
+set -xe
+
+# Just run everything in ci-test-only-normal.sh, but with reference processing enabled.
+export MMTK_NO_REFERENCE_TYPES=false
+. $(dirname "$0")/ci-test-only-normal.sh
diff --git a/.github/workflows/run-dacapo-2006.yml b/.github/workflows/run-dacapo-2006.yml
index 5985afe0..c353bf44 100644
--- a/.github/workflows/run-dacapo-2006.yml
+++ b/.github/workflows/run-dacapo-2006.yml
@@ -45,7 +45,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        test-script: ["ci-test-only-normal", "ci-test-only-normal-no-compressed-oops"]
+        test-script: ["ci-test-only-normal", "ci-test-only-normal-no-compressed-oops", "ci-test-only-weak-ref"]
     steps:
       # We need to check out the binding repo to get the running configs and test scripts
       - name: Checkout MMTk OpenJDK binding
diff --git a/DONT_MERGE b/DONT_MERGE
new file mode 100644
index 00000000..e69de29b
diff --git a/mmtk/Cargo.lock b/mmtk/Cargo.lock
index c74bc311..fe214905 100644
--- a/mmtk/Cargo.lock
+++ b/mmtk/Cargo.lock
@@ -459,7 +459,7 @@ dependencies = [
 [[package]]
 name = "mmtk"
 version = "0.31.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=7d798ad8a71aa6df80f58f5e565e4f9274d06871#7d798ad8a71aa6df80f58f5e565e4f9274d06871"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=d93262b4397df4866a64a1d268689ae20f208713#d93262b4397df4866a64a1d268689ae20f208713"
 dependencies = [
  "atomic",
  "atomic-traits",
@@ -497,7 +497,7 @@ dependencies = [
 [[package]]
 name = "mmtk-macros"
 version = "0.31.0"
-source = "git+https://github.com/mmtk/mmtk-core.git?rev=7d798ad8a71aa6df80f58f5e565e4f9274d06871#7d798ad8a71aa6df80f58f5e565e4f9274d06871"
+source = "git+https://github.com/mmtk/mmtk-core.git?rev=d93262b4397df4866a64a1d268689ae20f208713#d93262b4397df4866a64a1d268689ae20f208713"
 dependencies = [
  "proc-macro-error",
  "proc-macro2",
diff --git a/mmtk/Cargo.toml b/mmtk/Cargo.toml
index 6a33a938..3140683d 100644
--- a/mmtk/Cargo.toml
+++ b/mmtk/Cargo.toml
@@ -35,7 +35,7 @@ probe = "0.5"
 # - change branch
 # - change repo name
 # But other changes including adding/removing whitespaces in commented lines may break the CI.
-mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "7d798ad8a71aa6df80f58f5e565e4f9274d06871" }
+mmtk = { git = "https://github.com/mmtk/mmtk-core.git", rev = "d93262b4397df4866a64a1d268689ae20f208713" }
 # Uncomment the following to build locally
 # mmtk = { path = "../repos/mmtk-core" }
 
diff --git a/openjdk/mmtkUpcalls.cpp b/openjdk/mmtkUpcalls.cpp
index fd90cf0e..c665a191 100644
--- a/openjdk/mmtkUpcalls.cpp
+++ b/openjdk/mmtkUpcalls.cpp
@@ -87,6 +87,15 @@ static void mmtk_resume_mutators(void *tls) {
     MutexLockerEx locker(MMTkHeap::heap()->gc_lock(), Mutex::_no_safepoint_check_flag);
     MMTkHeap::heap()->gc_lock()->notify_all();
   }
+
+  log_debug(gc)("Notifying mutators blocking on Heap_lock for reference pending list...");
+  // Note: That's the ReferenceHandler thread.
+  {
+    MutexLockerEx x(Heap_lock, Mutex::_no_safepoint_check_flag);
+    if (Universe::has_reference_pending_list()) {
+      Heap_lock->notify_all();
+    }
+  }
 }
 
 static const int GC_THREAD_KIND_WORKER = 1;
@@ -293,6 +302,8 @@ static void mmtk_enqueue_references(void** objects, size_t len) {
     return;
   }
 
+  MutexLocker x(Heap_lock);
+
   oop first = (oop) objects[0]; // This points to the first node of the linked list.
   oop last = first; // This points to the last node of the linked list.
 
@@ -313,6 +324,7 @@ static void mmtk_enqueue_references(void** objects, size_t len) {
 
   oop old_first = Universe::swap_reference_pending_list(first);
   HeapAccess<AS_NO_KEEPALIVE>::oop_store_at(last, java_lang_ref_Reference::discovered_offset, old_first);
+  assert(Universe::has_reference_pending_list(), "Reference pending list is empty after swap");
 }
 
 OpenJDK_Upcalls mmtk_upcalls = {
diff --git a/openjdk/mmtkVMOperation.cpp b/openjdk/mmtkVMOperation.cpp
index 7ba6053f..51227ee3 100644
--- a/openjdk/mmtkVMOperation.cpp
+++ b/openjdk/mmtkVMOperation.cpp
@@ -32,21 +32,8 @@ VM_MMTkSTWOperation::VM_MMTkSTWOperation(MMTkVMCompanionThread *companion_thread
     _companion_thread(companion_thread) {
 }
 
-bool VM_MMTkSTWOperation::doit_prologue() {
-    Heap_lock->lock();
-    return true;
-}
-
 void VM_MMTkSTWOperation::doit() {
     log_trace(vmthread)("Entered VM_MMTkSTWOperation::doit().");
     _companion_thread->do_mmtk_stw_operation();
     log_trace(vmthread)("Leaving VM_MMTkSTWOperation::doit()");
 }
-
-void VM_MMTkSTWOperation::doit_epilogue() {
-    // Notify the reference processing thread
-    if (Universe::has_reference_pending_list()) {
-        Heap_lock->notify_all();
-    }
-    Heap_lock->unlock();
-}
diff --git a/openjdk/mmtkVMOperation.hpp b/openjdk/mmtkVMOperation.hpp
index 49897ce7..98d37ea4 100644
--- a/openjdk/mmtkVMOperation.hpp
+++ b/openjdk/mmtkVMOperation.hpp
@@ -39,9 +39,7 @@ class VM_MMTkSTWOperation : public VM_MMTkOperation {
 
 public:
   VM_MMTkSTWOperation(MMTkVMCompanionThread *companion_thread);
-  virtual bool doit_prologue() override;
   virtual void doit() override;
-  virtual void doit_epilogue() override;
 };
 
 #endif // MMTK_OPENJDK_MMTK_VM_OPERATION_HPP