Gisi{rr-97-05 a Partial Approach to the Problem of Deadlocks in Processes with Resources

In the modeling and control of manufacturing systems one of the aims is to obtain a system that can run in an automatic way. This means that the software in charge of the monitoring and control of the system must be able to deal with a set of problematic situations (system failures, system deadlocks, over ow of storing facilities, etc.). In this paper we are going to concentrate on one of the previous problems: the deadlock problem. Deadlock problems are very usual in concurrent system where, even if the execution of each process is correct, the competition of a set of processes for the same set of resources can give rise to deadlocks. This problem is very common to all the environments where processes share resources (or communicate via messages): databases, operating systems, manufacturing systems, etc. The present work focuses on the deadlock problem for a class of systems that appears in Flexible Manufacturing Systems (FMS) environments. We are going to identify a class of nets that appears frequently in this environment; for this class (called here ES3PR) we characterize deadlock problems to be related to structural elements of the Petri net model (siphons). We use this characterization in order to establish a control policy for deadlock prevention.