Linear-Time Logic. Hao Zheng - PDF Free Download

Linear-Time Logic Hao Zheng Department of Computer Science and Engineering University of South Florida Tampa, FL 33620 Email: zheng@cse.usf.edu Phone: (813)974-4757 Fax: (813)974-5456 Hao Zheng (CSE, USF) Comp Sys Verification 1 / 37

Overview 1 Linear Time Logic: Syntax & Semantics (Section 5.1.1-5.1.3) 2 Linear Time Logic: Equivalences (Section 5.1.4) 3 Linear Time Logic: Additional Operators (Section 5.1.5) 4 Linear Time Logic: Specifying Fairness (Section 5.1.6) Hao Zheng (CSE, USF) Comp Sys Verification 2 / 37

LT Properties An LT property is a set of infinite traces over AP. Specifying such sets explicitly is often inconvenient. Mutual exclusion is specified over AP = { c 1, c 2 } by P mutex = set of infinite words A 0 A 1 A 2... with { c 1, c 2 } A i for all i 0 Starvation freedom is specified over AP = { c 1, w 1, c 2, w 2 } by P nostarve = set of infinite words A 0 A 1 A 2... such that: ( ) ( ) ( ) ( ) j. w1 A j j. c1 A j j. w2 A j j. c2 A j Such properties can be specified succinctly using linear temporal logic. Hao Zheng (CSE, USF) Comp Sys Verification 3 / 37

Contents 1 Linear Time Logic: Syntax & Semantics (Section 5.1.1-5.1.3) 2 Linear Time Logic: Equivalences (Section 5.1.4) 3 Linear Time Logic: Additional Operators (Section 5.1.5) 4 Linear Time Logic: Specifying Fairness (Section 5.1.6) Hao Zheng (CSE, USF) Comp Sys Verification 4 / 37

5.1.1 Linear Temporal Logic (LTL): Syntax Linear temporal logic is a logic for describing LT properties. An extension of propositional logic with temporal modalities. Modal logic over infinite sequences [Pnueli 1977]. Propositional logic: a AP atomic proposition φ and φ ψ negation and conjunction Temporal operators: φ next state fulfills φ φ U ψ φ holds Until a ψ-state is reached Syntax of LTL over AP ϕ ::= true a ϕ ϕ ϕ ϕ ϕ U ϕ where a AP is an atomic proposition. Hao Zheng (CSE, USF) Comp Sys Verification 5 / 37

LTL Derived Operators φ ψ ( φ ψ) φ ψ φ ψ φ ψ (φ ψ) (ψ φ) φ ψ (φ ψ) ( φ ψ) true φ φ false true φ true U φ eventually in the future φ φ globally true Precedence order: The unary operators bind stronger than the binary ones. and bind equally strong. U takes precedence over,, and. Hao Zheng (CSE, USF) Comp Sys Verification 6 / 37

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary arbitrary a (next step) a b a Ub (until) a arbitrary arbitrary arbitrary a b a b b arbitrary a (eventually) a a a a arbitrary a (globally) a a a a a Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 7/ 1 Hao Zheng (CSE, USF) Comp Sys Verification 7 / 37

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary arbitrary a arbitrary arbitrary arbitrary Let σ = A a (next step) 0 A 1 A 2... (2 AP ) ω. σ = a iff a A a b a b a 0 (i.e., A b 0 = a) b arbitrary a Ub (until) a (eventually) a a a a arbitrary a (globally) a a a a a Hao Chris Zheng J. Myers (CSE, (Lecture USF) 5: LTL) Verification Comp of Sys Cyber-Physical Verification Systems 7 / 7/ 37 1

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary a (next step) arbitrary a arbitrary arbitrary arbitrary Let σ = A 0 A 1 Aa 2.. b. (2 AP a ) b. a b b arbitrary a Ub (until) σ = a iff A 1 = a a a a a arbitrary a (eventually) a (globally) a a a a a Hao Chris Zheng J. Myers (CSE, (Lecture USF) 5: LTL) Verification Comp of Sys Cyber-Physical Verification Systems 7 / 7/ 37 1

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary arbitrary a (next step) a b a Ub (until) a arbitrary arbitrary arbitrary a b a b b arbitrary Let σ = A 0 A 1 A 2 a... (2 AP a ) ω. a a arbitrary a (eventually) σ = a U b iff j 0. A j = b and A i = a, 0 i < j a a a a a a (globally) Hao Chris Zheng J. Myers (CSE, (Lecture USF) 5: LTL) Verification Comp of Sys Cyber-Physical Verification Systems 7 / 7/ 37 1

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary arbitrary a (next step) a b a Ub (until) a arbitrary arbitrary arbitrary a b a b b arbitrary a (eventually) a a a a arbitrary Let σ = A 0 A 1 A 2. a.. (2 AP a) ω. a a a a (globally) σ = a iff i 0. A i = a Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 7/ 1 Hao Zheng (CSE, USF) Comp Sys Verification 7 / 37

LTL Intuitive Semantics LTL Intuitive Semantics a (atomic prop.) a arbitrary arbitrary arbitrary arbitrary a (next step) arbitrary a arbitrary arbitrary arbitrary a b a b a b b arbitrary a Ub (until) Let σ = A 0 A 1 A 2... (2 AP ) ω. a a a a arbitrary a (eventually) σ = a iff i 0. A i = a a (globally) a a a a a Hao Chris Zheng J. Myers (CSE, (Lecture USF) 5: LTL) Verification Comp of Sys Cyber-Physical Verification Systems 7 / 7/ 37 1

New Temporal Modalities and Let σ = A 0 A 1 A 2... (2 AP ) ω. ϕ infinitely often ϕ......... σ = ϕ iff i 0 j i. A j = ϕ Hao Zheng (CSE, USF) Comp Sys Verification 8 / 37

New Temporal Modalities and Let σ = A 0 A 1 A 2... (2 AP ) ω. ϕ eventually forever ϕ... σ = ϕ iff i 0 j i. A j = ϕ Hao Zheng (CSE, USF) Comp Sys Verification 9 / 37

Traffic Light Properties Once red, the light cannot become green immediately (red green) The light becomes green eventually: The light becomes green infinitely often: Once red, the light becomes green eventually: green green (red green) Once red, the light always becomes green eventually after being yellow for some time in-between: (red (red U (yellow (yellow U green)))) Note these properties assume European traffic light which goes red, red/yellow, green, yellow, repeat. Hao Zheng (CSE, USF) Comp Sys Verification 10 / 37

LTL General Semantics (5.1.2) Let σ = A 0 A 1 A 2... (2 AP ) ω. σ = true σ = a iff a A 0 (i.e., A 0 = a) σ = ϕ 1 ϕ 2 iff σ = ϕ 1 and σ = ϕ 2 σ = ϕ iff σ = ϕ σ = ϕ iff σ[1..] = A 1 A 2 A 3... = ϕ σ = ϕ 1 U ϕ 2 iff j 0. σ[j..] = ϕ 2 and σ[i..] = ϕ 1, 0 i < j where σ[i..] = A i A i+1 A i+2... is suffix of σ from index i on. Hao Zheng (CSE, USF) Comp Sys Verification 11 / 37

General Semantics of,, and Let σ = A 0 A 1 A 2... (2 AP ) ω. σ = ϕ iff j 0. σ[j..] = ϕ σ = ϕ iff j 0. σ[j..] = ϕ σ = ϕ iff j 0. i j. σ[i...] = ϕ σ = ϕ iff j 0. i j. σ[i...] = ϕ where σ[i..] = A i A i+1 A i+2... is suffix of σ from index i on. Hao Zheng (CSE, USF) Comp Sys Verification 12 / 37

Definition 5.6 Semantics Over Words The LT-property { induced by LTL formula ϕ over AP is: Words(ϕ) = σ (2 AP) ω } σ = ϕ, where = is the smallest satisfaction relation. Hao Zheng (CSE, USF) Comp Sys Verification 13 / 37

Definition 5.7 Semantics Over Paths and States Let TS = (S, Act,, I, AP, L) be a transition system without terminal states, and let ϕ be an LTL-formula over AP. For infinite path fragment π of TS: For state s S: π = ϕ iff trace(π) = ϕ s = ϕ iff π Paths(s). π = ϕ TS satisfies ϕ, denoted TS = ϕ, iff Traces(TS) Words(ϕ) Hao Zheng (CSE, USF) Comp Sys Verification 14 / 37

Semantics for Transition Systems TS = ϕ iff (* transition system semantics *) Traces(TS) Words(ϕ) iff (* definition of = for LT-properties *) TS = Words(ϕ) iff (* Definition of Words(ϕ) *) π = ϕ for all π Paths(TS) iff (* semantics of = for states *) s 0 = ϕ for all s 0 I. Hao Zheng (CSE, USF) Comp Sys Verification 15 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a? TS = a? Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a TS = a? TS = (a b)? Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a TS = (a TS = a? b) TS = ( b (a b))? Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a TS = TS (a = a? b) TS = ( b (a b)) Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a TS = TS (a = a? b) TS = ( b (a b)) TS = b U (a b)? Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

LTL Examples LTL Examples s 1 {a,b } s 2 {a,b } s 3 {a} TS = a TS = TS (a = a? b) TS = ( b (a b)) TS = b U (a b) Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 12 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 16 / 37

Semantics of Negation For paths, it holds π = ϕ if and only if π = ϕ since: Words( ϕ) = ( 2 AP) ω \ Words(ϕ). But: TS = ϕ and TS = ϕ are not equivalent in general. It holds: TS = ϕ implies TS = ϕ, not always the reverse! Note that: TS = ϕ iff Traces(TS) Words(ϕ) iff Traces(TS) \ Words(ϕ) iff Traces(TS) Words( ϕ). TS neither satisfies ϕ nor ϕ if there are paths π 1 and π 2 in TS such that π 1 = ϕ and π 2 = ϕ. Hao Zheng (CSE, USF) Comp Sys Verification 17 / 37

Negation Example Negation Example s 1 s 2 {a} s 0 /0 /0 A transition system for which TS = a and TS = a. A transition system for which TS 6 = a and TS 6 = a. Hao Zheng (CSE, USF) Comp Sys Verification 18 / 37

5.1.4 Equivalence LTL formulas φ, ψ are equivalent, denoted φ ψ, if: Words(φ) = Words(ψ) Hao Zheng (CSE, USF) Comp Sys Verification 20 / 37

Duality and Idempotence Laws Duality: φ φ φ φ φ φ Idempotency: φ φ φ φ φ U (φ U ψ) φ U ψ (φ U ψ) U ψ φ U ψ Hao Zheng (CSE, USF) Comp Sys Verification 21 / 37

Absorption and Distributive Laws Absorption: φ φ φ φ Distribution: (φ U ψ) ( φ) U ( ψ) (φ ψ) φ ψ (φ ψ) φ ψ but......: (φ U ψ) ( φ) U ( ψ) (φ ψ) φ ψ (φ ψ) φ ψ Hao Zheng (CSE, USF) Comp Sys Verification 22 / 37

Distributive Laws Distributive Laws {b } {a} /0 TS 6 = (a ^ b) and TS = a ^ b TS = (a b) and TS = ( a b) Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 18 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 23 / 37

5.1.5 Weak Until The weak-until (or: unless) operator: ϕ W ψ def = (ϕ U ψ) ϕ ϕ W ψ does not require a ψ-state to be reached. Until U and weak until W are dual: (ϕ U ψ) (ϕ ψ) W ( ϕ ψ) (ϕ W ψ) (ϕ ψ) U ( ϕ ψ) Until and weak until are equally expressive: ψ ψ W false ϕ U ψ (ϕ W ψ) ψ Hao Zheng (CSE, USF) Comp Sys Verification 25 / 37

The Release Operator The release operator: ϕ R ψ def = ( ϕ U ψ). ψ always holds, a requirement that is released as soon as ϕ holds.......... Hao Zheng (CSE, USF) Comp Sys Verification 26 / 37

Recall Action-Based Fairness Constraints For set A of actions and infinite run ρ: Unconditional fairness Some action in A occurs infinitely often along ρ. Strong fairness If actions in A are infinitely often enabled (not necessarily always!) then some action in A has to occur infinitely often in ρ. Weak fairness If actions in A are continuously enabled (no temporary disabling!) then it has to occur infinitely often in ρ. This chapter uses state-based fairness assumptions (and constraints). Hao Zheng (CSE, USF) Comp Sys Verification 28 / 37

5.1.6 LTL Fairness Constraints Let Φ and Ψ be propositional logic formulas over AP. 1 An unconditional LTL fairness constraint is of the form: ufair = Ψ 2 A strong LTL fairness condition is of the form: sfair = Φ Ψ 3 A weak LTL fairness constraint is of the form: wfair = Φ Ψ Φ stands for something is enabled ; Ψ for something is taken Hao Zheng (CSE, USF) Comp Sys Verification 29 / 37

Fair Satisfaction For state s in transition system TS (over AP) without terminal states, let FairPaths fair (s) = { π Paths(s) π = fair } FairTraces fair (s) = { trace(π) π FairPaths fair (s) } For LTL-formula ϕ, and LTL fairness assumption fair: s = fair ϕ if and only if π FairPaths fair (s). π = ϕ and TS = fair ϕ if and only if s 0 I. s 0 = fair ϕ = fair is the fair satisfaction relation for LTL; = the standard one for LTL Hao Zheng (CSE, USF) Comp Sys Verification 30 / 37

Example 5.27 Randomized Arbiter noncrit 1 unlock noncrit 2 req 1 req 2 rel wait 1 head rel tail rel wait 2 enter 1 enter 2 crit 1 enter 1 lock enter 2 crit 2 TS 1 k Arbiter k TS 2 6 = = crit 11 But: TS 1 karbiter k TS 2 = fair crit crit 1 1 ^ crit crit 2 2 with fair = head head ^ tail tail Hao Zheng (CSE, USF) Comp Sys Verification 31 / 37

Semaphore-Based Mutual Exclusion Semaphore-Based Mutual Exclusion n 1,n 2,y=1 rel req 2 req 1 rel w 1,n 2,y=1 n 1,w 2,y=1 req 2 enter 2 enter 1 req 1 c 1,n 2,y=0 w 1,w 2,y=1 n 1,c 2,y=0 req rel 2 rel enter 2 enter1 req 1 c 1,w 2,y=0 w 1,c 2,y=0 F = /0, {enter 1 },{enter 2 }, {req 1 },{req 2 } sfair 1 = (wait 1 ^ crit 2 )! crit 1 sfair 1 = wait 1 crit 1 Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 35 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 32 / 37

Semaphore-Based Semaphore-Based Mutual Exclusion Mutual Exclusion n 1,n 2,y=1 rel req 2 req 1 rel w 1,n 2,y=1 n 1,w 2,y=1 req 2 enter 2 enter 1 req 1 c 1,n 2,y=0 w 1,w 2,y=1 n 1,c 2,y=0 req rel 2 rel enter 2 enter1 req 1 c 1,w 2,y=0 w 1,c 2,y=0 F = /0, {enter 1 },{enter 2 }, {req 1 },{req 2 } sfair 1 = (wait 1 ^ crit 2 )! crit 1 fair = sfair 1 sfair 2 Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 35 / 1 Hao Zheng (CSE, USF) Comp Sys Verification 32 / 37

Semaphore-Based Mutual Exclusion Semaphore-Based Mutual Exclusion n 1,n 2,y=1 rel req 2 req 1 rel w 1,n 2,y=1 n 1,w 2,y=1 req 2 enter 2 enter 1 req 1 c 1,n 2,y=0 w 1,w 2,y=1 n 1,c 2,y=0 req rel 2 rel enter 2 enter1 req 1 c 1,w 2,y=0 w 1,c 2,y=0 F = /0, {enter 1 },{enter 2 }, {req 1 },{req 2 } sfair 1 = (wait 1 ^ crit 2 )! crit 1 fair = sfair 1 sfair 2 Chris J. Myers (Lecture 5: LTL) Verification of Cyber-Physical Systems 35 / 1 TS Sem = fair crit 1 crit 2 Hao Zheng (CSE, USF) Comp Sys Verification 32 / 37

Theorem 5.30 Reducing = fair to = For: A transition system TS without terminal states LTL formula ϕ, and LTL fairness assumption fair It holds: TS = fair ϕ if and only if TS = (fair ϕ) Verifying an LTL-formula under a fairness assumption can be done using standard verification algorithms for LTL. Hao Zheng (CSE, USF) Comp Sys Verification 33 / 37

LTL Model-Checking Problem The following decision problem: Given finite transition system TS and LTL-formula ϕ: yields yes if TS = ϕ, and no (plus a counterexample) if TS = ϕ See section 5.2 for details. Hao Zheng (CSE, USF) Comp Sys Verification 34 / 37

A First Attempt TS = ϕ if and only if Traces(TS) Words(ϕ) }{{} L ω(a ϕ) if and only if Traces(TS) L ω (A ϕ ) = But complementation of NBA is quadratically exponential. If A has n states, A has c n2 states in worst case! Use the fact that L ω (A ϕ ) = L ω (A ϕ )! Hao Zheng (CSE, USF) Comp Sys Verification 35 / 37

Observation TS = ϕ if and only if Traces(TS) Words(ϕ) if and only if if and only if if and only if Traces(TS) ( (2 AP ) ω \ Words(ϕ) ) = Traces(TS) Words( ϕ) = }{{} L ω(a ϕ) TS A ϕ = F where F is the set of accepting states of A ϕ. LTL model checking is thus reduced to persistence checking! Hao Zheng (CSE, USF) Comp Sys Verification 36 / 37

Overview ofoverview LTL Model of LTL Checking Model Checking System Negation of property Model of system LTL-formula j model checker Generalised Büchi automaton G j Transition system TS Büchi automaton A j Product transition system TS A j TS A j = P pers(a j) Yes No (counter-example) Hao Zheng (CSE, USF) Comp Sys Verification 37 / 37