diff --git a/pcbnew/pad.cpp b/pcbnew/pad.cpp
index 177c21dadf..729d6c54a3 100644
--- a/pcbnew/pad.cpp
+++ b/pcbnew/pad.cpp
@@ -299,6 +299,13 @@ int PAD::GetBoundingRadius() const
 
 void PAD::BuildEffectiveShapes( PCB_LAYER_ID aLayer ) const
 {
+    std::lock_guard<std::mutex> RAII_lock( m_shapesBuildingLock );
+
+    // If we had to wait for the lock then we were probably waiting for someone else to
+    // finish rebuilding the shapes.  So check to see if they're clean now.
+    if( !m_shapesDirty )
+        return;
+
     BOARD* board = GetBoard();
     int    maxError = board ? board->GetDesignSettings().m_MaxError : ARC_HIGH_DEF;
 
diff --git a/pcbnew/pad.h b/pcbnew/pad.h
index 9fc3ddcdb7..24a67f23a6 100644
--- a/pcbnew/pad.h
+++ b/pcbnew/pad.h
@@ -655,6 +655,7 @@ private:
 
     // Must be set to true to force rebuild shapes to draw (after geometry change for instance)
     mutable bool                              m_shapesDirty;
+    mutable std::mutex                        m_shapesBuildingLock;
     mutable int                               m_effectiveBoundingRadius;
     mutable EDA_RECT                          m_effectiveBoundingBox;
     mutable std::shared_ptr<SHAPE_COMPOUND>   m_effectiveShape;