diff --git a/src/db/db/dbRecursiveInstanceIterator.cc b/src/db/db/dbRecursiveInstanceIterator.cc
index 2b9efb3d0..e06a5f09b 100644
--- a/src/db/db/dbRecursiveInstanceIterator.cc
+++ b/src/db/db/dbRecursiveInstanceIterator.cc
@@ -61,6 +61,7 @@ RecursiveInstanceIterator &RecursiveInstanceIterator::operator= (const Recursive
 
     m_box_convert = d.m_box_convert;
 
+    m_locker = d.m_locker;
     m_inst = d.m_inst;
     m_inst_array = d.m_inst_array;
     m_empty_cells_cache = d.m_empty_cells_cache;
@@ -175,7 +176,7 @@ RecursiveInstanceIterator::set_region (const box_type &region)
 {
   if (m_region != region || mp_complex_region.get () != 0) {
     init_region (region);
-    m_needs_reinit = true;
+    reset ();
   }
 }
 
@@ -183,7 +184,7 @@ void
 RecursiveInstanceIterator::set_region (const region_type &region)
 {
   init_region (region);
-  m_needs_reinit = true;
+  reset ();
 }
 
 void
@@ -196,7 +197,7 @@ RecursiveInstanceIterator::confine_region (const box_type &region)
   } else {
     init_region (m_region & region);
   }
-  m_needs_reinit = true;
+  reset ();
 }
 
 void
@@ -209,7 +210,7 @@ RecursiveInstanceIterator::confine_region (const region_type &region)
   } else {
     init_region (region & region_type (m_region));
   }
-  m_needs_reinit = true;
+  reset ();
 }
 
 void
@@ -218,7 +219,7 @@ RecursiveInstanceIterator::enable_all_targets ()
   if (! m_all_targets) {
     m_all_targets = true;
     m_targets.clear ();
-    m_needs_reinit = true;
+    reset ();
   }
 }
 
@@ -228,7 +229,7 @@ RecursiveInstanceIterator::set_targets (const std::set<db::cell_index_type> &tgt
   if (m_all_targets || m_targets != tgt) {
     m_targets = tgt;
     m_all_targets = false;
-    m_needs_reinit = true;
+    reset ();
   }
 }
 
@@ -264,6 +265,8 @@ RecursiveInstanceIterator::validate (RecursiveInstanceReceiver *receiver) const
   m_needs_reinit = false;
 
   //  re-initialize
+  m_locker = db::LayoutLocker ();
+
   mp_cell = mp_top_cell;
   m_trans_stack.clear ();
   m_inst_iterators.clear ();
@@ -310,6 +313,17 @@ RecursiveInstanceIterator::validate (RecursiveInstanceReceiver *receiver) const
     next_instance (receiver);
 
   }
+
+  if (mp_layout && ! at_end ()) {
+    m_locker = db::LayoutLocker (const_cast <db::Layout *> (mp_layout.get ()), true);
+  }
+}
+
+void
+RecursiveInstanceIterator::reset ()
+{
+  m_needs_reinit = true;
+  m_locker = db::LayoutLocker ();
 }
 
 void 
@@ -320,7 +334,7 @@ RecursiveInstanceIterator::reset_selection ()
     m_start.clear ();
     m_stop.clear ();
 
-    m_needs_reinit = true;
+    reset ();
 
   }
 }
@@ -335,7 +349,7 @@ RecursiveInstanceIterator::unselect_cells (const std::set<db::cell_index_type> &
       m_start.erase (*c);
     }
 
-    m_needs_reinit = true;
+    reset ();
 
   }
 }
@@ -350,7 +364,7 @@ RecursiveInstanceIterator::unselect_all_cells ()
       m_stop.insert (c->cell_index ());
     }
 
-    m_needs_reinit = true;
+    reset ();
 
   }
 }
@@ -365,7 +379,7 @@ RecursiveInstanceIterator::select_cells (const std::set<db::cell_index_type> &ce
       m_stop.erase (*c);
     }
 
-    m_needs_reinit = true;
+    reset ();
 
   }
 }
@@ -380,7 +394,7 @@ RecursiveInstanceIterator::select_all_cells ()
       m_start.insert (c->cell_index ());
     }
 
-    m_needs_reinit = true;
+    reset ();
 
   }
 }
@@ -441,6 +455,7 @@ void
 RecursiveInstanceIterator::next (RecursiveInstanceReceiver *receiver)
 {
   if (! at_end ()) {
+
     ++m_inst_array;
     if (! m_inst_array.at_end ()) {
       new_inst_member (receiver);
@@ -449,6 +464,12 @@ RecursiveInstanceIterator::next (RecursiveInstanceReceiver *receiver)
       new_inst (receiver);
     }
     next_instance (receiver);
+
+    if (at_end ()) {
+      //  Take this opportunity the release the layout lock.
+      //  This way, the shape iterator can be held further, without blocking the layout.
+      m_locker = db::LayoutLocker ();
+    }
   }
 }
 
diff --git a/src/db/db/dbRecursiveInstanceIterator.h b/src/db/db/dbRecursiveInstanceIterator.h
index dd06c5cf5..5b746fbf3 100644
--- a/src/db/db/dbRecursiveInstanceIterator.h
+++ b/src/db/db/dbRecursiveInstanceIterator.h
@@ -141,7 +141,7 @@ public:
   { 
     if (m_max_depth != depth) {
       m_max_depth = depth; 
-      m_needs_reinit = true;
+      reset ();
     }
   }
 
@@ -164,7 +164,7 @@ public:
   {
     if (m_min_depth != depth) {
       m_min_depth = depth;
-      m_needs_reinit = true;
+      reset ();
     }
   }
 
@@ -262,17 +262,14 @@ public:
   {
     if (m_overlapping != f) {
       m_overlapping = f;
-      m_needs_reinit = true;
+      reset ();
     }
   }
 
   /**
    *  @brief Reset the iterator
    */
-  void reset () 
-  {
-    m_needs_reinit = true;
-  }
+  void reset ();
 
   /**
    *  @brief Gets the selected target cells
@@ -552,6 +549,7 @@ private:
   std::unique_ptr<region_type> mp_complex_region;
   db::box_convert<db::CellInst> m_box_convert;
 
+  mutable db::LayoutLocker m_locker;
   mutable inst_iterator m_inst;
   mutable inst_array_iterator m_inst_array;
   mutable instance_element_type m_combined_instance;
diff --git a/src/db/db/dbRecursiveShapeIterator.cc b/src/db/db/dbRecursiveShapeIterator.cc
index 27d86e2aa..d9994162f 100644
--- a/src/db/db/dbRecursiveShapeIterator.cc
+++ b/src/db/db/dbRecursiveShapeIterator.cc
@@ -511,7 +511,14 @@ RecursiveShapeIterator::validate (RecursiveShapeReceiver *receiver) const
   }
 }
 
-void 
+void
+RecursiveShapeIterator::reset ()
+{
+  m_needs_reinit = true;
+  m_locker = db::LayoutLocker ();
+}
+
+void
 RecursiveShapeIterator::reset_selection ()
 {
   if (mp_layout) {
@@ -554,13 +561,6 @@ RecursiveShapeIterator::unselect_all_cells ()
   }
 }
 
-void
-RecursiveShapeIterator::reset ()
-{
-  m_needs_reinit = true;
-  m_locker = db::LayoutLocker ();
-}
-
 void 
 RecursiveShapeIterator::select_cells (const std::set<db::cell_index_type> &cells)
 {
diff --git a/src/db/unit_tests/dbRecursiveInstanceIteratorTests.cc b/src/db/unit_tests/dbRecursiveInstanceIteratorTests.cc
index eb06a6967..92ff90e4d 100644
--- a/src/db/unit_tests/dbRecursiveInstanceIteratorTests.cc
+++ b/src/db/unit_tests/dbRecursiveInstanceIteratorTests.cc
@@ -790,3 +790,69 @@ TEST(6)
   );
 }
 
+//  layout locking
+TEST(7_LayoutLocking)
+{
+  db::Layout layout;
+
+  layout.insert_layer (0);
+
+  db::Cell &c0 (layout.cell (layout.add_cell ()));
+  db::Cell &c1 (layout.cell (layout.add_cell ()));
+
+  db::Box b (0, 100, 1000, 1200);
+  c1.shapes (0).insert (b);
+
+  db::Trans tt;
+  c0.insert (db::CellInstArray (db::CellInst (c1.cell_index ()), tt));
+  c0.insert (db::CellInstArray (db::CellInst (c1.cell_index ()), db::Trans (db::Vector (2000, -2000))));
+
+  EXPECT_EQ (layout.under_construction (), false);
+
+  db::RecursiveInstanceIterator iter (layout, c0);
+
+  EXPECT_EQ (layout.under_construction (), false);
+
+  EXPECT_EQ (iter.at_end (), false);
+  EXPECT_EQ (layout.under_construction (), true);
+
+  EXPECT_EQ (iter.instance ().to_string (), "cell_index=1 r0 *1 0,0");
+  EXPECT_EQ (layout.under_construction (), true);
+  ++iter;
+
+  EXPECT_EQ (iter.at_end (), false);
+
+  EXPECT_EQ (iter.instance ().to_string (), "cell_index=1 r0 *1 2000,-2000");
+  EXPECT_EQ (layout.under_construction (), true);
+  ++iter;
+
+  EXPECT_EQ (layout.under_construction (), false);
+  EXPECT_EQ (iter.at_end (), true);
+
+  //  reset will restart
+  iter.reset ();
+
+  EXPECT_EQ (layout.under_construction (), false);
+
+  EXPECT_EQ (iter.at_end (), false);
+  EXPECT_EQ (layout.under_construction (), true);
+
+  //  a copy will hold the lock
+  iter.reset ();
+
+  EXPECT_EQ (layout.under_construction (), false);
+  EXPECT_EQ (iter.at_end (), false);
+
+  EXPECT_EQ (layout.under_construction (), true);
+  db::RecursiveInstanceIterator iter_copy = iter;
+
+  while (! iter.at_end ()) {
+    ++iter;
+  }
+
+  EXPECT_EQ (layout.under_construction (), true);
+  iter_copy = db::RecursiveInstanceIterator ();
+
+  EXPECT_EQ (layout.under_construction (), false);
+}
+