Atomic instead of volatile to match counter

src/engine/net/client/handlers/ItemProductionMsgHandler.java

@@ -150,7 +150,7 @@ public class ItemProductionMsgHandler extends AbstractClientMsgHandler {
             ForgeManager.itemWorkOrderLookup.remove(virtualItem);
             DbManager.removeFromCache(virtualItem);
 
-            workOrder.slots_used.set(workOrder.slots_used.get() - 1);
+            workOrder.slots_used.decrementAndGet();
 
             // Update workOrder on disk
 
@@ -429,7 +429,7 @@ public class ItemProductionMsgHandler extends AbstractClientMsgHandler {
 
             // Slot is no longer allocated to this workOrder.
 
-            workOrder.slots_used.set(workOrder.slots_used.get() - 1);
+            workOrder.slots_used.decrementAndGet();
 
             // Update workOrder on disk