infinite state model checking with presburger arithmetic constraints tevfik bultan department of...
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Infinite State Model Checking with Presburger Arithmetic Constraints
Tevfik BultanDepartment of Computer Science
University of California, Santa Barbara
Joint Work with My Students
• Action Language Verifier– Tuba Yavuz-Kahveci (PhD 2004) – Constantinos Bartzis (PhD 2004)
• Design for verification– Aysu Betin-Can (PhD 2005)
Infinite State Model Checking?
• Model checking started as a finite state verification technique
• Advantages of finite state systems:– Exhaustive state enumeration is possible for finite state
systems
• Disadvantages of infinite state systems:– Verification problems that are computable for finite state
systems are uncomputable for infinite state systems
Why Care About Infinity?
• Computer systems do not have infinite memory or infinite time– So why care about infinity?
• Infinity is an abstraction– Abstraction is at the core of computer science
• Computers are built with layers of abstractions– Abstraction is necessary for design– Abstraction is necessary for analysis
Why Care About Infinity?
• Reason 1:– Enables us to check a specification with respect to an
arbitrarily large number of components or memory• For example, arbitrary number of threads
• Reason 2: – Rather than developing verification techniques that rely
on the bound of the state space to terminate• Enables us to develop infinite state verification
techniques that terminate independent of the bound• A technique which is guaranteed to terminate is not
helpful if it runs out of memory
An Example
• A simple example that demonstrates limitations of (finite state) model checkers
• Property P can be verified with an infinite state model checker that uses standard backward fixpoint computations
• Fixpoint computation for some properties – for example, AG(State1 x 6)
may not converge but we can use conservative approximations
State0State0 State1State1
x’=x+1x’=x+1
x’=x+1x’=x+1
Initial: x=0 Initial: x=0 State0 State0
P: AG(State1 P: AG(State1 x is odd) x is odd)
P: AG(State1 P: AG(State1 ( ( . x =2 . x =2+1))+1))
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Symbolic Model Checking[McMillan et al. LICS 1990]
• Represent sets of states and the transition relation as Boolean logic formulas
• Forward and backward fixpoints can be computed by iteratively manipulating these formulas– Forward, backward image: Existential variable
elimination– Conjunction (intersection), disjunction (union) and
negation (set difference), and equivalence check• Use an efficient data structure for manipulation of Boolean
logic formulas– BDDs
Symbolic Model Checking
• What do you need to compute fixpoints? Symbolic Conjunction(Symbolic,Symbolic) Symbolic Disjunction(Symbolic,Symbolic) Symbolic Negation(Symbolic) BooleanEquivalenceCheck(Symbolic,Symbolic) Symbolic Precondition(Symbolic)
• Precondition (i.e., EX) computation is handled by: – variable renaming, followed by conjunction, followed by
existential variable elimination
• Infinite state model checking: Use a symbolic representation that is capable of representing infinite sets and supports the above functionality
Linear Arithmetic Constraints
• Linear arithmetic formulas can represent (infinite) sets of valuations of unbounded integers
• Linear integer arithmetic formulas on can be stored as a set of polyhedra
where each is a linear equality or inequality constraint is a linear equality or inequality constraint
and each is a polyhedronand each is a polyhedron
xxii integer variable, a integer variable, aii coefficient, c constant coefficient, c constant
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∧l
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∧l
klf
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ia ⋅ix
i=1
v
∑ =c
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A Linear Arithmetic Constraint Manipulator
• Omega Library [Pugh et al.]
– A tool for manipulating Presburger arithmetic formulas: First order theory of integers without multiplication
– Equality and inequality constraints are not enough
– Divisibility constraints are also needed
• which means: is divisible by ∑=
⋅v
iii xa
1
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∃y(ia ⋅
ixi=1
v
∑ = y⋅c)
c
2y x – 1
x – 5 2y
3y x + 7
x 3y
dark shadow
real shadow
293
y
x
Integers are Complicated
Presburger Arithmetic Model Checking[Bultan et al. CAV’97, TOPLAS’99]
• Use linear arithmetic constraints as a symbolic representation
• Use a Presburger arithmetic manipulator as the symbolic engine (Omega library)
• Compute fixpoints to verify or falsify CTL properties
• Use conservative approximations to achieve convergence
What About Using BDDs for Encoding Arithmetic Constraints?
• Arithmetic constraints on bounded integer variables can be represented using BDDs
• Use a binary encoding
– represent integer x as x0x1x2... xk
– where x0, x1, x2, ... , xk are binary variables
– You have to be careful about the variable ordering!
• BDDs and constraint representations are both applicable– Which one is better?
Arithmetic Constraints vs. BDDs[Bultan TACAS’00]
Arithmetic Constraints vs. BDDs
Arithmetic Constraints vs. BDDs
• Constraint based verification can be more efficient than BDDs for integers with large domains
• Constraint based verification can be used to automatically verify infinite state systems– cannot be done using BDDs
• However, BDD-based verification is more robust and the arithmetic constraint representation has two problems:
• Problem 1: Constraint based verification does not scale well when there are boolean or enumerated variables in the specification
• Problem 2: Price of infinity– CTL model checking becomes undecidable for infinite
domains
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Conservative Approximations
• Compute a lower ( p ) or an upper ( p+ ) approximation to the truth set of the property ( p )
Conservative Approximations
• Compute a lower ( p ) or an upper ( p+ ) approximation to the truth set of the property ( p )
• There are three possible outcomes:
II pppp
1) “The property is satisfied”
Conservative Approximations
• Compute a lower ( p ) or an upper ( p+ ) approximation to the truth set of the property ( p )
• There are three possible outcomes:
II pppp
1) “The property is satisfied”
II pp
3) “I don’t know”
2) “The property is false and here is a counter-example”
II pp ppsates whichsates whichviolate the violate the propertyproperty
pp++
pp
Conservative Approximations
• Compute a lower ( p ) or an upper ( p+ ) approximation to the truth set of the property ( p )
• There are three possible outcomes:
II pppp
1) “The property is satisfied”
II pp
3) “I don’t know”
2) “The property is false and here is a counter-example”
II pp ppsates whichsates whichviolate the violate the propertyproperty
pp++
pp
Conservative Approximations
• Truncated fixpoint computations– To compute a lower bound for a least-fixpoint
computation – Stop after a fixed number of iterations
• Widening– To compute an upper bound for the least-fixpoint
computation– We use a generalization of the polyhedra widening
operator by [Cousot and Halbwachs POPL’78]
y 5
1 y
x 4
x y
y
x
Polyhedra Widening
Ai: x y x 4 y 5 1 y
y 5
1 y
x 4
x y
y
x
Polyhedra Widening
x y
y 5
1 y
x 5
Ai: x y x 4 y 5 1 y
Ai+1: x y x 5 y 5 1 y
y 5
1 y
x 4
x y
y
x
Polyhedra Widening
x 5
Ai: x y x 4 y 5 1 y
Ai+1: x y x 5 y 5 1 y
AiAi+1: x y y 5 1 y
y 5
1 y
x 4
x y
y
x
Polyhedra Widening
x y
y 5
1 y
x 5
Ai: x y x 4 y 5 1 y
Ai+1: x y x 5 y 5 1 y
AiAi+1: x y y 5 1 y
Ai Ai+1 is defined as:
all the constraints in Ai
that are also satisfied by Ai+1
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Automata Representation for Arithmetic Constraints [Bartzis, Bultan CIAA’02, IJFCS ’02]
• Given an atomic linear arithmetic constraint in one of the following two forms
we can construct an FA which accepts all the solutions to the given constraint
• By combining such automata one can handle full Presburger arithmetic
cxav
iii=∑
=
⋅1
i ii
v
a x c⋅ <=∑
1
Basic Construction
• We first construct a basic state machine which– Reads one bit of each variable at each step, starting
from the least significant bits– and executes bitwise binary addition and stores the
carry in each step in its state
0 1 2
0 10 0/ /0 1
01/ 1
0 11 1/ / 0 1
0 10 0/ /0 1
11 /1
1 1 / 0
00/1
Examplex + 2y
010+ 2 001
10 0
01/ 0
10 /0
)(1
||∑=
v
iiaONumber of states:
In my figures alphabet symbols are written vertically!
Automaton Construction
• Equality With 0– All transitions writing 1 go to a sink state– State labeled 0 is the only accepting state– For disequations (), state labeled 0 is the only
rejecting state• Inequality (<0)
– States with negative carries are accepting– No sink state
• Non-zero Constant Term c– Same as before, but now -c is the initial state– If there is no such state, create one (and possibly some
intermediate states which can increase the size by |c|)
Conjunction and Disjunction
0 0 10,1,1
0 10,1
10
10
10
01
0 0 10,1,1
Automaton for x-y<1-1
0 1
0 00,1
0 10,1
0 1 11,0,1
1 10,1
01
10
Automatonfor 2x-y>0
0
-1
-2
11
01
10
00
0 00,1
11
10
10
01
0 10,1
0 11,1
10
00
00
0 10,1
10
01
0 11,1 1
0
Automaton for x-y<1 2x-y>0-1,-
1
0,-1
-2,-1
-1,0
-2,0
-2,1
• Conjunction and disjunction is handled by generating the product automaton
Other Extensions
• Existential quantification (necessary for pre and post)– Project the quantified variables away– The resulting FA is non-deterministic
• Determinization may result in exponential blowup of the FA size but we do not observe this in practice
– For universal quantification use negation
• Constraints on all integers– Use 2’s complement arithmetic– The basic construction is the same– In the worst case the size doubles
Experiments
• We implemented these algorithms using MONA [Klarlund et al]
• We integrated them to our infinite state model checker
• We compared our automata representation against– the polyhedral representation used in the Omega library– the automata representation used in LASH [Boigelot and
Wolper]
• we also integrated LASH to our model checker by writing a wrapper around it
Experimental results
Construction time
0.01
0.1
1
10
100
1000
barbermp-1barbermp-2barbermp-3bakery2-1bakery3-1bakery4-1
ticket2-1ticket3-1ticket4-1
coherence-3coherence-4
pc5pc10 rw32rw64 sis1sis3
lightcontrolinsertionsort
problem instance
time (seconds)
Omega
Our constructionbased on MONALASH
Experimental results
Verification time
0.01
0.1
1
10
100
1000
barbermp-1barbermp-2barbermp-3bakery2-1bakery3-1bakery4-1
ticket2-1ticket3-1ticket4-1
coherence-3coherence-4
pc5pc10 rw32rw64 sis1sis3
lightcontrolinsertionsort
problem instance
time (seconds)
Omega
Our constructionbased on MONALASH
Experimental results
Memory comsumption
0.01
0.1
1
10
100
barbermp-1barbermp-2barbermp-3bakery2-1bakery3-1bakery4-1
ticket2-1ticket3-1ticket4-1
coherence-3coherence-4
pc5pc10 rw32rw64 sis1sis3
lightcontrolinsertionsort
problem instance
memory (Mbytes)
Omega
Our constructionbased on MONALASH
Efficient Pre- and Post-condition Computations [Bartzis, Bultan CAV’03]
• Pre and post condition computations can cause an exponential blow-up in the size of the automaton in the worst case
• We do not observe this blow-up in the experiments
• We proved that for a common class of systems this blow up does not occur
Assumptions About the Transition Relation
• We assume that the transition relation of the input system is a disjunction of formulas in the following form
guard(R) update(R)
where– guard(R) is a Presburger formula on current state
variables and– update(R) is of the form
xi’=f(x1, …, xv) xj’= xj
• In asynchronous concurrent systems the transition relation is usually in the above form
ji
Three Classes of Updates
1. xi’ = c
2. xi’ = xi + c
3. xi’ = j=1aj· xj + c
We proved that
1. Computation of pre is polynomial for all 3 cases
2. Computation of post is polynomial for 2 and for 3, whenever ai is odd.
v
Other Results Related to Automata Encoding[Bartzis, Bultan TACAS’03, STTT]
• We developed efficient BDD construction algorithms and proved bounds for the sizes of the BDDs for bounded linear arithmetic constraints– Given a linear arithmetic formula that contains n atomic
constraints on v bounded integer variables represented with b-bits, the size of the BDD is:
• These results explain why all three versions of SMV
(NuSMV, CMU SMV and Cadence SMV) are inefficient in handling linear arithmetic constraints – In SMV the BDD size could be exponential in b
€
O(vbj=1
n
∏ |ai, j
i=1
v
∑ |)
Other Results Related to Automata Encoding[CAV’04]
• We defined a widening operator for the automata representation of arithmetic constraints
• The widening operator looks for similar states in two consecutive iterations (Ai and Ai+1) and creates an equivalence relation– then it merges the states in the same equivalence class
• We can prove that for some cases this widening operator computes the exact fixpoint – for example for updates of the form x’=x+c
Example
• The sequence y=x, y=x y=x+1, y=x y=x+1 y=x+2, … does not converge
• However we know that a fixpoint exists (yx) and is representable as an arithmetic constraint
module incr_1 integer y; parameterized integer x;
initial: y=x;
incr_1: y'=y+1; spec: AG(y>=x)endmodule
Widening
• Instead of computing a sequence A1, A2, … where
Ai+1=Aipost(Ai)
compute A’1, A’2, … where
A’i+1=A’i(A’ipost(A’i))
• By definition AB AB
• The goal is to find a widening operator such that:– The sequence A’1, A’2, … converges– It converges fast– The computed fixpoint is as close as possible to the
exact set of reachable states
Widening Arithmetic Automata
• Given automata A and A’ we want to compute AA’
• We say that states k and k’ are equivalent (kk’) if either– k and k’ can be reached from either initial state with the
same string (unless k or k’ is a sink state)– or, the languages accepted from k and k’ are equal– or, for some state k’’, kk’’ and k’k’’
• The states of AA’ are the equivalence classes of
Example
0 10,1
1 00,1
XX
0 1
y=x
0 X0,1
0
1
2
3
0 10,1
0 10,1
1 00,1
10
10
01
XX
y=x y=x+1
0
1
0
1
2
3
0 1
3
Example
0 10,1
1 00,1
XX
0 1
0 X0,1
0
1
2
3
0 10,1
0 10,1
1 00,1
10
10
01
XX
=0 X0,1
10
10
0,1 201
3
XX
0 10,1
Example
00,
10
10
0 101
2
XX
0 10,1
X0
0
00
0 X0,1
2
1
10
3 4
XX
X1
10
01
10
01
0 10,1
10
0
1
2
0
1
2
3
4
1
0
2
3
4
X1
Example
X0
0
00
0 X0,1
2
1
10
3 4
XX
X1
10
01
10
01
0 10,1
10
00,
10
10
0 101
2
XX
0 10,1
X1
0 X0,1
10
X 10,0
0,2 1,301
= Represents:yx
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Composite Model Checking[Bultan, Gerber, League ISSTA 98, TOSEM 00]
• Map each variable type to a symbolic representation– Map boolean and enumerated types to BDD
representation– Map integer type to a linear arithmetic constraint
representation
• Use a disjunctive representation to combine different symbolic representations: composite representation
• Each disjunct is a conjunction of formulas represented by different symbolic representations– we call each disjunct a composite atom
Composite Representation
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F =i=1
n
∨f1i∧ f
2i∧...∧ f
ti
symbolic rep. 1
symbolic rep. 2
symbolic rep. t
composite atom
Example:
x: integer, y: boolean
x>0 and x´x-1 and y´ or x<=0 and x´x and y´y
arithmetic constraintrepresentation
BDD arithmetic constraintrepresentation
BDD
Composite Symbolic Library [Yavuz-Kahveci, Tuncer, Bultan TACAS01], [Yavuz-Kahveci, Bultan FroCos 02, STTT 03]
• Uses a common interface for each symbolic representation
• Easy to extend with new symbolic representations
• Enables polymorphic verification
• Multiple symbolic representations:– As a BDD library we use Colorado University Decision
Diagram Package (CUDD) [Somenzi et al] – As an integer constraint manipulator we use Omega
Library [Pugh et al]
Composite Symbolic Library Class Diagram
CUDD Library OMEGA Library
Symbolic+intersect()
+union()+complement()+isSatisfiable()+isSubset()+pre()+post()
CompSym
–representation: list of comAtom
+intersect()+ union() • • •
BoolSym
–representation: BDD
+intersect()+union() • • •
IntSym
–representation: Polyhedra
+intersect()+union() • • •
compAtom
–atom: *Symbolic
Pre and Post-condition Computation
Variables:x: integer, y: boolean
Transition relation:R: x>0 and x´x-1 and y´ or x<=0 and x´x and y´y
Set of states: s: x=2 and !y or x=0 and !y
Compute post(s,R)
Pre and Post-condition Distribute
R: x>0 and x´x-1 and y´ or x<=0 and x´x and y´y
s: x=2 and !y or x=0 and y
post(s,R) = post(x=2 , x>0 and x´x-1) post(!y , y´) x=1 y
post(x=2 , x<=0 and x´x) post (!y , y´y) false !y
post(x=0 , x>0 and x´x-1) post(y , y´) false y
post (x=0 , x<=0 and x´x) post (y, y´y ) x=0 y
= x=1 and y or x=0 and y
Polymorphic Verifier
Symbolic TranSys::check(Node *f) {
•
•
•
Symbolic s = check(f.left)
case EX:
s.pre(transRelation)
case EF:
do
sold = s
s.pre(transRelation)
s.union(sold)
while not sold.isEqual(s) •
•
•
}
Action Language Verifier Action Language Verifier is polymorphicis polymorphic
It becomes a BDD based model It becomes a BDD based model checker when there or no integer checker when there or no integer variablesvariables
Composite Representation + Shape Graphs[Yavuz-Kahveci, Bultan SAS 02]
• Shape graphs represent the states of the heap
• Each node in the shape graph represents a dynamically allocated memory location
• Heap variables point to nodes of the shape graph • The edges between the nodes show the locations pointed
by the fields of the nodes
addadd toptop
nextnext
nextnextn1n1 n2n2
heap variables heap variables addadd and and toptoppoint to node n1point to node n1
add.nextadd.next is node n2 is node n2top.nexttop.next is also node n2 is also node n2
add.next.nextadd.next.next is is nullnull
Composite Symbolic Library: Further Extended
CUDD Library OMEGA Library
Symbolic+union()
+isSatisfiable()+isSubset()+forwardImage()
CompSym
–representation: list of comAtom
+ union() • • •
BoolSym
–representation: BDD
+union() • • •
compAtom
–atom: *Symbolic
HeapSym
–representation: list of ShapeGraph
+union() • • •
IntSym
–representation: list of Polyhedra
+union() • • •
ShapeGraph
–atom: *Symbolic
Forward Fixpoint
pc=l1 mutex numItems=2
addaddtoptop
pc=l2 mutex numItems=2 addadd toptop
BDDBDD arithmetic constraintarithmetic constraintrepresentationrepresentation
A set of shape graphsA set of shape graphs
pc=l4 mutex numItems=2 addadd toptop
pc=l1 mutex numItems=3 addadd toptop
Post-condition Computation: Example
pc=l4 mutex numItems=2 addadd toptop
pc=l4 and mutex’pc’=l1
pc=l1 mutex
numItems’=numItems+1
numItems=3
top’=add
addadd toptop
set ofset ofstatesstates
transitiontransitionrelationrelation
Again: Fixpoints Do Not Converge
• We have two reasons for non-termination– integer variables can increase without a bound– the number of nodes in the shape graphs can increase
without a bound
• As I mentioned earlier, we use widening on integer variables to achieve convergence
• For heap variables we use the summarization operation
Summarization Example
pc=l1 mutex numItems=3
addaddtoptop
pc=l1 mutex numItems=3 summarycount=2
addaddtoptop
summary nodesummary nodea new integer variablea new integer variablerepresenting the numberrepresenting the numberof concrete nodes encoded of concrete nodes encoded by the summary nodeby the summary node
After summarization, it becomes:After summarization, it becomes:
summarized nodessummarized nodes
Simplification
pc=l1 mutex numItems=3
summaryCount=2
addadd toptop
pc=l1 mutex
addaddtoptop numItems=4
summaryCount=3
==
pc=l1 mutex
addaddtoptop (numItems=4
summaryCount=3
numItems=3
summarycount=2)
Simplification On the Integer Part
pc=l1 mutex
addaddtoptop
(numItems=4
summaryCount=3
numItems=3
summaryCount=2)
==
pc=l1 mutex
addaddtoptop numItems=summaryCount+1
3 numItems
numItems 4
Then We Use Integer Widening
pc=l1 mutex
addaddtoptop numItems=summaryCount+1
3 numItems
numItems 4
pc=l1 mutex
addaddtoptop numItems=summaryCount+1
3 numItems
numItems 5
pc=l1 mutex
addaddtoptop numItems=summaryCount+1
3 numItems
==
Now, fixpoint convergesNow, fixpoint converges
Verified Properties
Specification Verified Invariants
Stack top=null numItems=0
topnull numItems0
numItems=2 top.nextnull
Single Lock Queue head=null numItems=0
headnull numItems0
(head=tail head null) numItems=1
headtail numItems0
Two Lock Queue numItems>1 headtail
numItems>2 head.nexttail
Verifying Linked Lists with Multiple Fields
• Pattern-based summarization– User provides a graph grammar rule to describe the
summarization pattern
L x = next x y, prev y x, L y
• Represent any maximal sub-graph that matches the pattern with a summary node– no node in the sub-graph pointed by a heap variable
Summarization Pattern Examples
......nn nn nnL L x x xx.n = .n = yy, L, L y y
......nn nn nnL L x x xx.n = .n = yy, , yy.p = .p = xx, L, L y y
pp pp pp
L L x x xx.n = .n = yy, , xx.d = .d = zz, L, L y y ......nn nn nn
dd dd dd
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Action Language Tool Set
Action LanguageAction LanguageParserParser
Action LanguageAction LanguageVerifier (ALV)Verifier (ALV)
Omega Omega LibraryLibrary
CUDDCUDDPackagePackage MONAMONA
Composite Symbolic LibraryComposite Symbolic Library
PresburgerPresburgerArithmeticArithmeticManipulatorManipulator
BDDBDDManipulatorManipulator
AutomataAutomataManipulatorManipulator
Action LanguageAction LanguageSpecificationSpecification
VerifiedVerified Counter Counter exampleexample
Action Language [Bultan, ICSE 00], [Bultan, Yavuz-Kahveci, ASE 01]
• Variables: boolean, enumerated, integer (unbounded)• Parameterized constants
– specifications are verified for all possible values• Transition relation is defined using actions and modules
– Atomic actions: Predicates on current and next state variables
– Action composition: • asynchronous (|) or synchronous (&)
– A module is defined as asynchronous and/or synchronous compositions of its actions and submodules
Readers Writers Example: A Closer Look
module main()integer nr;boolean busy;restrict: nr>=0;initial: nr=0 and !busy;
module Reader()boolean reading;initial: !reading;rEnter: !reading and !busy and nr’=nr+1 and reading’;rExit: reading and !reading’ and nr’=nr-1;Reader: rEnter | rExit;
endmodule
module Writer() ... endmodule
main: Reader() | Reader() | Writer() | Writer();spec: invariant(busy => nr=0)
endmodule
S S : Cartesian product of: Cartesian product of variable domains defines variable domains defines the set of statesthe set of states
I I : Predicates defining : Predicates defining the initial statesthe initial states
RR : Atomic actions of the : Atomic actions of the ReaderReader
RR : Transition relation of : Transition relation of Reader defined as Reader defined as asynchronous composition asynchronous composition of its atomic actionsof its atomic actions
RR : Transition relation of main defined as asynchronous : Transition relation of main defined as asynchronous composition of two Reader and two Writer processescomposition of two Reader and two Writer processes
Arbitrary Number of Threads
• Counting abstraction– Create an integer variable for each local state of a
thread– Each variable will count the number of threads in a
particular state • Local states of the threads have to be finite
– Specify only the thread behavior that relates to the correctness of the controller
– Shared variables of the controller can be unbounded• Counting abstraction can be automated
Readers-Writers After Counting Abstraction
module main()integer nr;boolean busy;
parameterized integer numReader, numWriter;restrict: nr>=0 and numReader>=0 and numWriter>=0;initial: nr=0 and !busy;module Reader()
integer readingF, readingT;initial: readingF=numReader and readingT=0;rEnter: readingF>0 and !busy and nr’=nr+1 and readingF’=readingF-1 and
readingT’=readingT+1;rExit: readingT>0 and nr’=nr-1 readingT’=readingT-1
and readingF’=readingF+1;Reader: rEnter | rExit;
endmodulemodule Writer()
...endmodulemain: Reader() | Writer();spec: invariant(busy => nr=0)
endmodule
Variables introduced by the counting abstractions
Parameterized constants introduced by the counting abstractions
Verification of Readers-Writers
Integers Booleans Cons. Time (secs.)
Ver. Time (secs.)
Memory (Mbytes)
RW-4 1 5 0.04 0.01 6.6
RW-8 1 9 0.08 0.01 7
RW-16 1 17 0.19 0.02 8
RW-32 1 33 0.53 0.03 10.8
RW-64 1 65 1.71 0.06 20.6
RW-P 7 1 0.05 0.01 9.1
Outline
• Model checking with arithmetic constraints• Conservative approximations• Automata representation for arithmetic constraints• Composite representation• Action Language Verifier (ALV)• Checking synchronization in concurrent programs with ALV
Design for Verification
Action Language
Verifier
Verification of Synchronization in
ConcurrentPrograms
Design forVerification
uses
enables
A Java Read-Write Lock Implementation
class ReadWriteLock { private Object lockObj; private int totalReadLocksGiven; private boolean writeLockIssued; private int threadsWaitingForWriteLock; public ReadWriteLock() { lockObj = new Object(); writeLockIssued = false; } public void getReadLock() { synchronized (lockObj) { while ((writeLockIssued) || (threadsWaitingForWriteLock != 0)) { try { lockObj.wait(); } catch (InterruptedException e) { } } totalReadLocksGiven++; } } public void getWriteLock() { synchronized (lockObj) { threadsWaitingForWriteLock++;
while ((totalReadLocksGiven != 0) || (writeLockIssued)) { try { lockObj.wait(); } catch (InterruptedException e) { // } } threadsWaitingForWriteLock--; writeLockIssued = true; } } public void done() { synchronized (lockObj) {
//check for errors if ((totalReadLocksGiven == 0) && (!writeLockIssued)) { System.out.println(" Error: Invalid call to release the lock"); return; } if (writeLockIssued) writeLockIssued = false; else totalReadLocksGiven--;
lockObj.notifyAll(); }
}
}
How do we translate this toAction Language?
Action Language
Verifier
Verification of Synchronization in
JavaPrograms
Design for Verification
• Abstraction and modularity are key both for successful designs and scalable verification techniques
• The question is: – How can modularity and abstraction at the design level be better
integrated with the verification techniques which depend on these principles?
• Our approach:– Structure software in ways that facilitate verification– Document the design decisions that can be useful for verification– Improve the applicability and scalability of verification using this
information
A Design for Verification Approach
We have been investigating a design for verification approach based on the following principles:
1. Use of design patterns that facilitate automated verification
2. Use of stateful, behavioral interfaces which isolate the behavior and enable modular verification
3. An assume-guarantee style modular verification strategy that separates verification of the behavior from the verification of the conformance to the interface specifications
4. A general model checking technique for interface verification
5. Domain specific and specialized verification techniques for behavior verification
• Avoids usage of error-prone Java synchronization primitives: synchronize, wait, notify
• Separates controller behavior from the threads that use the controller: Supports a modular verification approach which exploits this modularity for scalable verification
class Action{ protected final Object owner; … private boolean GuardedExecute(){ boolean result=false; for(int i=0; i<gcV.size(); i++) try{ if(((GuardedCommand)gcV.get(i)).guard()){
((GuardedCommand)gcV.get(i)).update(); result=true; break; }
}catch(Exception e){} return result; } public void blocking(){ synchronized(owner) { while(!GuardedExecute()) { try{owner.wait();} catch (Exception e){} } owner.notifyAll(); } } public boolean nonblocking(){ synchronized(owner) { boolean result=GuardedExecute(); if (result) owner.notifyAll(); return result; } }}
class RWController implements RWInterface{
int nR; boolean busy;final Action act_r_enter, act_r_exit;final Action act_w_enter, act_w_exit;RWController() { ... gcs = new Vector(); gcs.add(new GuardedCommand() { public boolean guard(){
return (nR == 0 && !busy);} public void update(){busy = true;}} ); act_w_enter = new Action(this,gcs);}public void w_enter(){ act_w_enter.blocking();}public boolean w_exit(){ return act_w_exit.nonblocking();}public void r_enter(){ act_r_enter.blocking();}public boolean r_exit(){ return act_r_exit.nonblocking();}}
Reader-Writer ControllerThis helper class is
provided.No need to rewrite it!
Controller Interfaces
• A controller interface defines the acceptable call sequences for the threads that use the controller
• Interfaces are specified using finite state machines
public class RWStateMachine implements RWInterface{ StateTable stateTable; final static int idle=0,reading=1, writing=2; public RWStateMachine(){ ...
stateTable.insert("w_enter",idle,writing); } public void w_enter(){ stateTable.transition("w_enter"); } ...}writing
reading
idle
r_enter
r_exit
w_exit
w_enter
Concurrent Program
ControllerClasses
ThreadThreadThread
Classes
Controller InterfaceMachine
ControllerBehaviorMachine
JavaPath Finder
ActionLanguage
Verifier
ThreadIsolation
ThreadClass
CountingAbstraction
InterfaceVerification
BehaviorVerification
Verification Framework
InterfaceMachine
Thr
ead
1
Thr
ead
2
Thr
ead
n
Thr
ead
1
Controller
SharedData
InterfaceMachine
Thr
ead
2
InterfaceMachine
Thr
ead
n
Thread Modular Interface Verification
Concurrent Program
Controller Behavior
Modular Behavior Verification
Modular Design / Modular Verification
Interface
Behavior Verification
• Analyzing properties (specified in CTL) of the synchronization policy encapsulated with a concurrency controller and its interface– Verify the controller properties assuming that the user
threads adhere to the controller interface
• Behavior verification with Action Language Verifier– We wrote a translator which translates controller classes
to Action Language– Using counting abstraction we can check concurrency
controller classes for arbitrary number of threads
Interface Verification
• A thread is correct with respect to an interface if all the call sequences generated by the thread can also be generated by the interface machine– Checks if all the threads invoke controller methods in the order
specified in the interfaces – Checks if the threads access shared data only at the correct
interface states
• Interface verification with Java PathFinder– Verify Java implementations of threads– Correctness criteria are specified as assertions
• Look for assertion violations• Assertions are in the StateMachine and SharedStub
– Performance improvement with thread Isolation
Thread Isolation: Part 1
• Interaction among threads
• Threads can interact with each other in only two ways:– invoking controller actions– invoking shared data methods
• To isolate the threads– Replace concurrency controllers with controller interface
state machines– Replace shared data with shared stubs
Thread Isolation: Part 2
• Interaction among a thread and its environment
• Modeling thread’s call to its environment with stubs– File I/O, updating GUI components, socket operations,
RMI call to another program• Replace with pre-written or generated stubs
• Modeling the environment’s influence on threads with drivers– Thread initialization, RMI events, GUI events
• Enclose with drivers that generate all possible events that influence controller access
Automated Airspace Concept
• Automated Airspace Concept by NASA researchers automates the decision making in air traffic control
• The most important challenge is achieving high dependability
• Automated Airspace Concept includes a failsafe short term conflict detection component called Tactical Separation Assisted Flight Environment (TSAFE)– It is responsible for detecting conflicts in flight plans of the
aircraft within 1 minute from the current time– Dependability of this component is even more important
than the dependability of the rest of the system– It should be a smaller, isolated component compared to
the rest of the system so that it can be verified
TSAFE
TSAFE functionality:1. Display aircraft position2. Display aircraft planned route3. Display aircraft future projected route trajectory4. Show conformance problems
Server
Computation
FlightDatabase
GraphicalClient
Client
<<RMI>>
21,057 lines of code with 87 classes
Radar feed<<TCP/IP>>
User
EventThread
Feed Parser
Timer
TSAFE Architecture
Behavior Verification Performance
RW 0.17 1.03
Mutex 0.01 0.23
Barrier 0.01 0.64
BB-RW 0.13 6.76
BB-Mutex 0.63 1.99
Controller Time(sec) Memory (MB) P-Time (sec) P-Memory (MB)
8.10 12.05
0.98 0.03
0.01 0.50
0.63 10.80
2.05 6.47
P denotes parameterized verification for arbitrary number of threads
Interface Verification Performance
Thread Time (sec) Memory (MB)
TServer-Main 67.72 17.08
TServer-RMI 91.79 20.31
TServer-Event 6.57 10.95
TServer-Feed 123.12 83.49
TClient-Main 2.00 2.32
TClient-RMI 17.06 40.96
TClient-Event 663.21 33.09
Fault Categories
• Concurrency controller faults– initialization faults (2) – guard faults (2)– update faults (6)– blocking/nonblocking faults (4)
• Interface faults– modified-call faults (8)– conditional-call faults
• conditions based on existing program variables (13)• conditions on new variables declared during fault
seeding (5)
Effectiveness in Finding Faults
• Created 40 faulty versions of TSAFE
• Each version had at most one interface fault and at most one behavior fault – 14 behavior and 26 interface faults
• Among 14 behavior faults ALV identified 12 of them– 2 uncaught faults were spurious
• Among 26 interface faults JPF identified 21 of them– 2 of the uncaught faults were spurious– 3 of the uncaught faults were real faults that were not
caught by JPF
Falsification Performance
TServer-RMI 29.43 24.74
TServer-Event 6.88 9.56
TServer-Feed 18.51 94.72
TClient-RMI 10.12 42.64
TClient-Event 15.63 12.20
Thread Time (sec) Memory (MB)
RW-8 0.34 3.26
RW-16 1.61 10.04
RW-P 1.51 5.03
Mutex-8 0.02 0.19
Mutex-16 0.04 0.54
Mutex-p 0.12 0.70
Concurrency Controller Time (sec) Memory (MB)
Conclusions
• Infinite state model checking is feasible– Enables verification of specifications with unbounded
variables– Enables verification of parameterized systems
• Infinite state verification techniques can help us in identifying more efficient finite state model checking techniques
• Building extensible tools is important!
Conclusions
• Application of automated verification techniques to real-world systems leads to re-thinking the design– Design for verification can lead to more effective
verification
• Integrating verification tools lead to more effective verification– In our case ALV and JPF
Conclusions
• We were able to use our design for verification approach based on design patterns and behavioral interfaces in different domains
• Use of domain specific behavior verification techniques has been very effective– Interface verification was the bottleneck
• Model checking research resulted in various verification techniques and tools which can be customized for specific classes of software systems
• Automated verification techniques can scale to realistic software systems using design for verification approach
THE END