Existing　policies　for　deadlock　control　are　mainly　based　on　siphons　due　to　their　ability　to　indicate　deadlocks,　and　can　be　used　as　a　powerful　tool　to　deal　with　deadlock　situations　in　flexible　manufacturing　systems.　In　order　to　avoid　deadlocks,　researchers　often　add　monitors　to　control　siphons.　This　may　result　in　redundant　monitors,　unnecessary　cost,　and　restriction　of　the　behavior　permissiveness.　For　example,　for　a　system　of　sequential　systems　with　shared　resources　((SR)-R-4),　the　existing　deadlock　control　policies　based　on　max,　max　or　max-controlled　siphons　tend　to　overly　restrict　the　behavior　of　a　controlled　system.　To　ensure　maximal　permissive　behavior　of　controlled　systems,　a　new　concept　of　siphon　controllability　named　W-control　is　defined　and　then　a　sufficient　and　necessary　condition　under　which　a　(WSPR)-P-3　is　live　if　all　its　siphons　are　W-controlled.　Examples　are　given　to　demonstrate　them.