Victor Khomenko Newcastle University

1. A Usable Reachability Analyser Victor Khomenko Newcastle University

2. Reachability analysis • Problem statement: check if there is a reachable state s satisfying a given predicate R(s) • Usually R specifies some undesirable situation, e.g. a deadlock, violation of mutual exclusion, violation of an assertion • If the system is a safe Petri net then R is a Boolean expression over the elementary predicates corresponding to the places, e.g.: p1 p2 + p1 p3 + p2 p3

3. How to specify properties? • Manual specification is tedious and error-prone • Automatic generation of formulae can be done only for a fixed set of standard properties; hence custom properties cannot be checked, even if they are just minor variations of standard properties • Users are often forced to implement generators for their custom properties (simple in theory, hard work in practice)

4. T7 P10 T2 P4 P12 P2 P7 P1 P14 P9 T10 T5 P6 T4 T6 T1 T9 P8 T8 T3 P3 P11 P13 P5 Example: Dining Philosophers T11 T12 P15 P16 p1 (p2 + p7)(p3 + p8)(p4 + p5) p6 p9 (p7 + p10)(p8 + p9)(p12 + p13) p14 p1 p9 (p15 + p16)

5. How to specify properties? • In this case can reduce to standard deadlock checking: • In general, such reductions may be difficult or not possible • It is a bad idea to make the user to modify the model or invent tricks P15 P16

6. Proposed solution Language Reach for specifying reachability properties: • custom properties can be easily and concisely specified • the model does not have to be modified in any way, in particular the model does not have to be translated into an input language of some model checker • almost any reachability analyser can be used as the back-end

7. Example: deadlock property Mathematical definition: Reach specification: forall t in TRANSITIONS { exists p in pre t { ~$p } } or simply forall t in TRANSITIONS { ~@t } taking care of proper termination: forall t in TRANSITIONS { ~@t } & (~$P"p15" | ~$P"p16")

8. Reachability analysis flow

9. Case studies: asynchronous circuits Asynchronous circuits are circuits without clocks Very attractive: the traditional synchronous (clocked) designs lack flexibility to cope with contemporary microelectronics challenges Notoriously difficult to design correctly Often specified using Signal Transition Graphs (STGs) – a class of labelled Petri nets

10. Example: VME Bus Controller Data Transceiver Device Bus d lds dsr VME Bus Controller ldtack dtack dtack- dsr+ lds+ d- lds- ldtack- ldtack+ dsr- dtack+ d+

11. Case studies: Consistency In each possible execution, the transitions representing the rising and falling edges of each signal must be correctly alternated between, always starting from the same edge (either rising or falling) exists s in SIGNALS { let Ts = tran s { $s & exists t in Ts s.t. is_plus t { @t } | ~$s & exists t in Ts s.t. is_minus t { @t } } }

12. Case studies: Output persistency A local signal (output or internal) should not be disabled by any other transition x+ a+ a+ x+ a+ b+ x+ y+ x+ a+ x+ x+ a+ a+ x+ OP violation ok ok OP violation ok ok

13. Case studies: Output persistency exists t1 in TRANSITIONS s.t. sig(t1) in LOCAL { @t1 & exists t2 in TRANSITIONS s.t. sig(t2)!=sig(t1) & |pre(t1)*(pre(t2)\post(t2))|!=0 { @t2 & forall t3 in tran(sig(t1))\{t1} s.t. |pre(t3)*(pre(t2)\post(t2))|=0 { exists p in pre(t3)\post(t2) { ~$p } } } } Intuitively, we are looking for a marking where t1 is disabled by t2, and after t2 fires, no transition with the same signal as t1 is enabled

14. Case studies: CSC States with the same encoding should enable the same local signals 10000 dtack- dsr+ 00100 00000 lds+ ldtack- ldtack- ldtack- dtack- dsr+ 10010 01100 01000 11000 ldtack+ lds- lds- lds- dtack- dsr+ 11010 01110 11010 M’’ M’ 01010 d+ d- dsr- dtack+ 01111 11111 11011

15. Case studies: CSC Generalised reachability property: check if there are reachable states s1,…,sk satisfying a given predicate R(s1,…,sk) forall s in SIGNALS { $s <-> $$s } & exists s in LOCAL { @s^@@s }

16. Case studies: arbiters r1+ g1+ r1- g1- … Arbiter g1 r1 rn+ gn+ rn- gn- … … gn rn r1+ g1+ r1- g1- … rn+ gn+ rn- gn- Traditional protocol Early protocol

17. Case studies: deadlock in arbiters The rising request transitions are not weakly fair, i.e. any state (except the initial one) enabling only such transitions is a deadlock The initial state has to be treated in a special way A minor variation of a standard property that renders standard deadlock checkers almost useless let requests = {T"ra+", T"rb+", T"rc+"} { forall t in TRANSITIONS\requests { ~@t } } & exists p in PLACES { $p ^ is_init p } let requests = TT "r[a-z]\\++\\(/[0-9]\\+\\)\\?" {

18. Case studies: mutual exclusion Mutual exclusion of signals rather than places let a = $S"ga", b = $S"gb", c = $S"gc" { a & b | b & c | a & c } Alternatively: threshold[2]($S"ga", $S"gb", $S"gc") With a regular expression: let grants = SS "g[a-z]\\+" { threshold[2] g in grants { $g } }

19. Case studies: mutual exclusion Traditional mutual exclusion does not hold for the early protocol threshold[2]($S"ra" & $S"ga", $S"rb" & $S"gb", $S"rc"&$S"gc") With a regular expression: let req = SS "r[a-z]\\+" { threshold[2] r in req { $r & $S("g" + (name r)[1..]) } }

20. Conclusion A solution to the problem of generating formulae expressing custom reachability properties has been proposed The usefulness of this method is demonstrated on several case studies The developed MPSAT tool is currently being used as the reachability analysis engine within the DesiJ and Workcraft tools

21. Future work • Extension to other formalisms is straightforward (general Petri nets, coloured Petri nets, products of automata, digital circuits, etc.) • Extension to other property classes is straightforward (e.g. add LTL or CTL modalities) • Share common subterms during expansion • Add more powerful constructs, such as recursive definitions and rewriting rules