Concurrency Specification

1. Concurrency Specification Aliasgar Rampurwala Aditya Garg

2. Outline • Issues in concurrent systems • Programming language support for concurrency • Concurrency analysis - A specification based approach • Concurrency and other formal methods • Deadlock Checker • Concurrency and architectures

3. Concurrency • Coexistence • Sharing of resources • Issues • Asynchronicity • Non-determinism • Solution • Locks • Results • Deadlock and starvation

4. Concurrency in various disciplines • Databases • Transaction serializability • Operating Systems • Multithreading • Electronic circuits • Flip flops • Real life • Gas station example

5. Concurrency in architecture implementations

6. PL support for concurrency - 1 • Fork and join constructs • Queue construct and the signal operation • Concurrent Pascal • The Java synchronized keyword

7. PL support for concurrency - 2 • Communicating sequential processes [CSP] • Producer command : consumer!m • Consumer command : producer?n • Guarded commands • <guard> —› <command-list> • guard : list of declarations, boolean expressions or an input command • alternative guarded command • [ G1 —› C1 ƀ G2 —›C2 ƀ …. ƀ Gn —› Cn]

8. From Specification to Implementation - 1

9. From Specification to Implementation - 2 Specification PhaseImplementation Phase • Easy to verify safety Difficult to verify and liveness safety and liveness • State spaces small State spaces and manageable large and unmanageable; testing difficult • Cost of correcting Cost of correcting flaws is low flaws is high

10. Specification-based model - 1 • Synchronizer construct • set of variables defining the state of shared resources • set of operations on these variables (with pre/post conditions) • set of invariants • safety conditions • liveness conditions

11. Specification-based model - 2 • Process construct • independent thread of execution • multiple processes coexist • control allocation/deallocation of synchronizer controlled resources • Example: • Web server : synchronizer • Web browser : process

12. Gas-station model

13. Gas-station model - Program spec

14. Gas station model - RSTG

15. Gas station model - Event expressions • Two customers trying to buy gas concurrently

16. Gas station model - Reachability graph • Identifies the states that can be reached by executing enabled operations in processes and synchronizers • Constructed from event expressions and RSTG • Nodes represent states of RSTG • Edges represent operations from event expressions • A deadlock occurs if the graph contains terminal nodes

17. Tool support for concurrency analysis • INCA(Inequality Necessary Condition Analysis) • checks properties of an architectural specification (ex. Mutual exclusion) • provides example executions that violate those properties • verifies that a modification removes the faults

18. Detecting a race condition • Customer1 pays before Customer2 but Customer2 takes up the hose before Customer1 thus getting the amount of gas purchased by Customer1

19. The INCA query

20. INCA results • INCA generates a system of inequalities based on the violation of properties specified by the query • a consistent inequality implies such a situation is possible • an inconsistent inequality implies such a situation is impossible

21. Features common with other formal methods • RSTG • Pre and post conditions • State invariants

22. Unique Features - 1 • Operation execution phases • Request phase • Enabled phase • Service phase • only one operation invocation can be in the service phase. • Terminate phase • Example: Fair scheduler: []<>enabled(o) -> <>service(o)

23. Unique Features - 2 • Separation of control resources from state variables • Event expressions help “walk through” the concurrency aspect • Semantics of allocation and deallocation • helpful in detecting deadlocks

24. Deadlock Checker • Performs checks on parallel programs written in CSP in order to prove freedom from deadlock • Takes in a network file(.net) that has been compiled from a CSP source file using a tool such as FDR • More information : • http://users.ox.ac.uk/~jeremy/Deadlock/

25. The Dining Philosophers Problem • 5 philosophers and 5 chopsticks • All philosophers keep thinking • When a philosopher feels hungry, he picks up the chopsticks closest to him, eats rice and keeps the chopsticks back • Deadlock : • When all philosophers grab their left chopstick simultaneously

26. The Dining Philosophers Problem

27. Architectures and concurrency • Component types: • Synchronizer • Process units • Connector • Synchronization connector

28. Synchronization Connector

29. A simple Binary Semaphore Connector in Java package architecture.connector; import java.lang; import architecture.framework; public class SyncConnector extends Connector { private boolean available; public SyncConnector() { available = true; } public void handle(Request r) { String messageName = r.getName(); if(messageName.equals("AccessResource")) { if(!available) { wait(); } if(available) { available = false; super.handle(r); } } }

30. A simple Binary Semaphore Connector in Java (contd) public void handle(Notification n) { String messageName = n.getName(); if(messageName.equals("ReleaseResource")) { available = true; notifyAll(); } } }

31. Conclusions and Discussion • Analysis of concurrent systems early in the development process reduces complexity and cost of correcting errors • A formal analysis will help detect deadlocks and starvation and also in direct code generation • Concurrency in software architectures can be represented in terms of CSPs