Java Race FinderChecking Java Programs for

Sequential ConsistencyTuba Yavuz-Kahveci

Fall 2013

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

What is Sequential Consistency? Program statements are executed according to

program order Each thread’s statements are executed according

to the program order in that thread’s code Write atomicity

Each read operation on a variable sees the most recent write operation on that variable

What is a Memory Model? Constrains the behavior of memory operations

What value can a read operation see? Example memory models

Sequential Consistency Easy to understand

Relaxed Consistency Models Relaxation of

Program order Write atomicity

Who Should Care? Programmers

Understanding how to achieve sequential consistency, if possible

Reasoning about correctness Compiler writers

Optimizing code within the restrictions of the memory model

Problem: Getting Multi-threaded Java Programs Right Important Questions Any Java Programmer Should

Ask Is my multithreaded program correctly

synchronized? Beware!!! Sequential consistency is not guaranteed for

incorrectly synchronized Java programs! If my multithreaded program is not correctly

synchronized, how can I fix it? If my multithreaded program is not correctly

synchronized for a good reason, should I still be worried?

Automated tool support is needed to answer these nontrivial questions

An Example: Peterson’s Mutual Exclusion Algorithm - Version 1

Initialization: flag[0] = flag[1] = turn = shared = 0 /* all non-volatile */

s1: flag[0] = 1;

Thread 1

Thread 2

s2: turn = 1;s3: while (flag[1] == 1 && turn == 1) { /*spin*/}s4: shared++; /* critical section */s5: flag[0] = 0;

s6: flag[1] = 1;s7: turn = 0;s8: while (flag[0] == 1 && turn == 0) { /*spin*/}s9: shared++; /* critical section */s10: flag[0] = 0;

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

What is Java Memory Model (JMM)? A relaxed memory model Sequential consistency is guaranteed only for

correctly synchronized programs For programs without data races

Incorrectly synchronized programs can show extra behavior that is not sequentially consistent Still subject to some safety rules

Synchronization Rules in Java Some synchronization actions and their relationship in Java:

Unlocking a monitor lock synchronizes with locking that monitor lock.

Writing a volatile variable synchronizes with reading of that variable.

Starting a thread synchronizes with the first action of that thread.

Final action in a thread synchronizes with any action of a thread that detects termination of that thread.

Initialization of a field synchronizes with the first access to the field in every thread.

In general a release action synchronizes with a matching acquire action.

Happens-Before Relation An action a1 happens-before action a2, a1 ≤hb

a2, due to one of the following: a1 comes before a2 according to program order: a1

≤po a2. a1 synchronizes with a2: a1 ≤sw a2. a1 happens-before a’ that happens-before a2:

Exists a’. a1 ≤hb a’ and a’ ≤hb a2 (transitivity).

Happens-before, ≤hb = ( ≤po U ≤sw )+ , is a partial-order on all actions in an execution.

Happens-before Consistency A read operation r can see results of a write

operation w provided that: r does not happen-before w: not (r ≤hb w). There is no intervening write operation: not (exists

w’. w r ≤hb w’ ≤hb r).

Anatomy of a Data Race Definition: If two actions a1 and a2 from different

threads access the same memory location loc, the actions are not ordered by happens-before and if one of the actions is a write, then there is a data race on loc.

Example:

≤hb ≤hb

≤hb

≤hb

Thread 1

Thread 2

Initialization: boolean done = false; /* non-volatile */

done = true; if (done)

// use result

Race on

done!!!

result = compute();

A Simple Fix A write to a volatile variable synchronizes with a

read of that variable. Example:

≤hb ≤hb

≤hb ≤hb

Thread 1

Thread 2

Initialization: volatile boolean done = false;

done = true; if (done)

// use result

result = compute();≤hb

≤hb

≤hb

Not in a race

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

Our Solutions/Contributions Is my multi-threaded program correctly synchronized?

Kim K., Yavuz-Kahveci T., Sanders B.Precise Data Race detection in Relaxed Memory Model using Heuristic based Model Checking [ASE Conf. 2009]

If my multi-threaded program is not correctly synchronized, how can I fix it?Kim K., Yavuz-Kahveci T., Sanders B. JRF-E: Using Model Checking to give Advice on Eliminating Memory Model-related Bugs [ASE Conf. 2010, ASE Journal 2012]

If my program is not correctly synchronized for a good reason, should I still be worried?Jin H., Yavuz-Kahveci T., Sanders B. Java Path Relaxer: Extending JPF for JMM-aware model checking [JPF Workshop]Jin H., Yavuz-Kahveci T., Sanders B. Java Memory Model-Aware Model Checking [TACAS 2012]

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

State/Snapshot of a Running Java Program

Values of Static

FieldsHeap

(objects)

Thread states

Bytecode for the Java program

JAVA VIRTUAL MACHINE

Model Checking Java Programs

Values of Static

FieldsHeap

(objects)

Thread states

Main ThreadThread1Thread2Thread3

…

Main ThreadThread2Thread1Thread3

…

Main ThreadThread3Thread2Thread1

…

Model Checking for Sequential Consistency

Java Race Finder (JRF)

Java Path Finder (JPF)

Multi-threaded Java

application

DataRace?

yes

no

• a model-checker for Javaprograms• checks for generalcorrectness properties• assumes sequentialconsistency• explores all possible thread interleaving

• extends JPF’s state representation to detect data races

Our Approach for Detecting Data Races

Algorithm:

for each execution path EPj=<a1, a2, …, an> of program P do

initialize happens-before relation for each action ai , i= 1 to n, do

let loc be the memory location ai accesses

if (it is safe (without a data race) for ai to access loc)

generate DATA RACE error

execute ai

update happens-before relation

Representing Happens-Before We define an h-function that captures the

happens-before relation in an implicit way. h: SyncAddr U Thread -> 2Addr .

SyncAddr: Volatile variables and locks Addr: Non-volatile variables

Is it safe for aj of thread ti to access loc? does h(ti ) contain loc?

Which variables can be safely accessed if acquire on s (with a matching release on s) is executed? h(s).

The h-function Initialization:

At the beginning there is only the main thread: h0 = λz.if z = main then static(P) else φ

Update: Executing an action updates the h-function: action(t, x) h = h’

h: h-function before executing action t: the thread the action belongs to x: synchronization variable (volatile or a lock) h’: the updated h function

Updating the h-functionaction an by thread t hn+1

write a volatile field v release(t,v) hn

read a volatile field v acquire(t, v) hn

lock the lock variable lck acquire(t, lck) hn

unlock the lock variable lck release(t,lck) hn

start thread t′ release(t,t′) hn

join thread t′ acquire(t, t′) hn

t′.isAlive() returns false acquire(t, t′) hn

write a non-volatile field x invalidate(t, x) hn

read a non-volatile field x hn

instantiate an object containing non-volatile fields fields and volatile fields volatiles new (t, fields , volatiles ) hn

Action Semantics Variables that can be safely accessed from thread t copied to the set for

synchronization variable x

release(t, x)h = h[x →h(t)∪h(x)] Variables in the set of synchronization variable x will now be safely accessed by

thread t

acquire(t, x)h = h[t →h(t)∪h(x)] Only thread t which changed x can safely access it.

invalidate(t, x) h = λz. if (t = z) then h(z) else h(z)\{x} The non-volatile fields of the newly created object can be safely accessed by the

thread who created it. The volatile fields are initialized to refer to empty sets.

new(t, fields, volatiles)h = λz. if (t = z) then h(t) ∪ fields else if (z ∈ volatiles) then{} else h(z)

Implementation of the h-function

How JRF extends JPF

Test ProgramsSources # of examples # of examples found

to have data racesTextbook by Herhily and Shavitz.

65 19

Amino Concurrent Building Blocks Library

10 9

Google Concurrent Data Structures Workshop.

12 10

Java Grande Forum Benchmark Suite

10 6

Webserver Simulator – Student Projects

28 7

Time Overhead of JRF

0 2 4 6 8 10 121

10

100

1000

JPFJRF

Test Programs

Tim

e (s

ecs)

Space Overhead of JRF

0 2 4 6 8 10 121

10

100

1000

JPFJRF

Test Programs

Mem

ory

(MB)

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

Finding the data race quickly

race

State space of a programinitialstate

Each path from initial state to a leaf state represents a separate execution.

race

race

Finding the data race using DFS

race

State space of a programinitialstate

Each path from initial state to a leaf state represents a separate execution.

race

race

DFScounter-example

Finding the data race using BFS

race

State space of a programinitialstate

Each path from initial state to a leaf state represents a separate execution.

race

race

BFScounter-example

Heuristic-Based Data Race Search Our goal is to reach a state that has a data race as

quick as possible. Assign a traversal priority to each program state based

on how close it may be to a racy state. Writes-First (WF): Prefer write statements to read

statements Watch-Written (WW): Prefer access to memory locations

recently written by another thread Avoid Release/Acquire (ARA): Avoid scheduling threads

that perform proper synchronization. Acquire-First (AF): Prefer acquire operations that do not

have a matching release operation.

An Example: Peterson’s Mutual Exclusion Algorithm - Version 1

Initialization: flag[0] = flag[1] = turn = shared = 0 /* all non-volatile */

s1: flag[0] = 1;

Thread 1

Thread 2

s2: turn = 1;s3: while (flag[1] == 1 && turn == 1) { /*spin*/}s4: shared++; /* critical section */s5: flag[0] = 0;

s6: flag[1] = 1;s7: turn = 0;s8: while (flag[0] == 1 && turn == 0) { /*spin*/}s9: shared++; /* critical section */s10: flag[0] = 0;

DFS vs Heuristic Search

s1: flag[0] = 1;

Thread 1

s2: turn = 1;

s3: while (flag[1] == 1 && turn == 1) { /*spin*/}s4: shared++; /* critical section */ s5: flag[0] =

0;Thread 2

s6: flag[1] = 1;s7: turn = 0;

Thread 1

s1: flag[0] = 1;s2: turn = 1;

Thread 2

s6: flag[1] = 1;s7: turn = 0;

Race!turn not in h(thread2)!

DFSSearchPath

Heuristic

Search Path

Experimental Results: Heuristic Search

Code(lines of code)

Search State Length

Time(sec)

Memory(MB)

DisBarrier(232)

DFSHeuristicBFS

109 792589

109 39 36

4 3256

53 46644

Moldyn(1252)

DFSHeuristicBFS

282118965127

2821

950>574*

231257

1014

579518988

DEQueue(334)

DFSHeuristicBFS

331930

2812

9

112

272631

BinaryStaticTreeBarrier (1910)

DFSHeuristicBFS

61137

2275

6152

>18*

79

2221

6686

986*: JPF ran out of memory

Outline The Problem: Getting Multithreaded Java

Programs Right Java Memory Model Our Solution: Java Race Finder

What is model checking anyway? Representing Happens-before Heuristic-based Search Code Modification Suggestions

What went wrong?

Thread 1

s1: flag[0] = 1;s2: turn = 1;

Thread 2

s6: flag[1] = 1;s7: turn = 0;

source

statement

manifest stateme

nt

removes turn from h(thread2)

accesses turn when turn is not in h(thread2)

How to fix it? Data races are due to absence of happens-before

relationship Suggest code modifications that will create

happens-before relationship between the source and manifest statements Change the variable to volatile Change the array to an atomic array Move the source statement to make use of existing

happens-before relationships due to transitivity Perform the same synchronization Change another variable to volatile to create

happens-before relationships due to transitivity

Change to atomic arrayThread 1

s1: flag[0] = 1;s2: turn = 1;

Thread 2

s6: flag[1] = 1;

source

statement

manifest stateme

nt

removes flag[1] from h(thread1)

Accesses flag[1] when flag[1] is not in h(thread1)

Change flag to atomic array

Peterson’s ME Alg.

turn and flag are volatile

s3: while (flag[1] == 1 && turn == 1) { /*spin*/}

Thread 1

An Example for move sourceInitialization: goFlag = false; volatile Data publish;

s1: r = new Data();

Thread 1

Thread 2

s2: publish = r;s3: r.setDesc(e);s4: goFlag = true;

t1: if (publish != null) {t2: while (!goFlag);t3: String s = publish.getDesc(); t4: assert(s.equals(“e”); }

• Updates published object after making the reference visible• Compiler may reorder s3 and s4

• May use the published object when it is in an inconsistent state

Move source statement

s1: r = new Data();

Thread 1

Thread 2

s2: publish = r;s3: r.setDesc(e);s4: goFlag = true;

t1: if (publish != null) {t2: while (!goFlag);

publish is volatile

goFlag is not volatile

source

statement

removes goFlag from h(thread2)

manifest stateme

nt

Accesses goFlag when goFlag is not in h(thread2)

Move s4 before s2

s4: goFlag = true;

An Example for perform the same synchronization operation

Initialization: int data; final Object lock = new Object();

s1: print (data);

Thread 1

Thread 2

t1: synchronized (lock) { /*lock*/t2: data = 1;t3: } /*unlock*/

• For every non-volatile variable v, acquireHistory(v) stores the set of safe accesses by thread t via a synchronization operation on s.• Thread2’s safe access on data is noted as an example behavior.

Perform that synchronized block

s1: print (data);

Thread 1

Thread 2

t1: synchronized (lock) { /*lock*/t2: data = 1;t3: } /*unlock*/

data is not volatile

Perform synchronized (lock) to

access data

source

statement

removes data from h(thread1)

manifest stateme

nt

Accesses data when data is not in h(thread1)s0: synchronized (lock) {

/*lock*/

s2: } /*unlock*/

An Example for change another to volatile

Initialization: int x; boolean done = false; /* both non-volatile*/

s1: x = 1;

Thread 1

Thread 2

t1: while (!done);t2: assert(x == 1);

s2: done = true;

• Potential data races both on x and done.• Should we really change both to x and done to

volatile?• Can we get away by changing only one?

Change other to volatile

s1: x = 1;

Thread 1

Thread 2

t1: while (!done);t2: assert(x == 1);

s2: done = true;

x and done are not volatile

source

statement

removes x from h(thread2)

manifest stateme

ntaccesses x when

x is not in h(thread2)

Change done to volatile

JRF-E: Eliminating Data Races JRF is configured to produce threshold # of

counter-example paths and write to a file JRF-E works on the output of JRF and analyzes the

counter-example paths to generate code modification suggestions

For each race reports intersection of suggestions on all the

relevant counter-example paths For each specific code modification suggestion

reports the frequency

JRF-E RESULT====================================================== data race #1jrf.hbset.util.HBDataRaceException . . .______________________________________________________ analyze counter example data race source statement : "putstatic" at simple/SimpleRace.java:64 : "x = 1;"data race manifest statement : "getstatic" at simple/SimpleRace.java:74: "assert (x==1);"

Change the field "simple.SimpleRace.x from INITIALIZER" to volatile.Change the field "simple.SimpleRace.done from INITIALIZER" to volatile.

______________________________________________________ advice from acquiring historyNONE====================================================== data race #2jrf.hbset.util.HBDataRaceException . . .______________________________________________________ analyze counter example data race source statement : "putstatic" at simple/SimpleRace.java:65 : "done = true;"data race manifest statement : "getstatic" at simple/SimpleRace.java:73: "while(!done) { /*spin*/ }"

Change the field "simple.SimpleRace.done from INITIALIZER" to volatile.

______________________________________________________ advice from acquiring historyNONE______________________________________________________ frequency of advice[1times] Change the field "simple.SimpleRace.x from INITIALIZER" to volatile.[2times] Change the field "simple.SimpleRace.done from INITIALIZER" to volatile.______________________________________________________ statisticJRF takes 0:0:1 to find 2 equivalent races with 9 counterexample traces.JRF-E takes 0:0:0 in 9 races analysis.

How did it

happen?

How many times a suggestion has

been made considering all the

races?

feedback ona single race

feedback on all races

How to fix it?

feedback onanother race

JRF-E - Analyzing threshold # of races

DisBarr

ier

LockFr

eeHash

Set

Optimist

icList

MCSLock*

Linea

rSense

Barrie

rVolat

ile

Iterat

or_EB

Deque

Lufact sor

Webser

werSim

0.11

10100

1000

Threshold=100Threshold=10Threshold=1

Tim

e (s

ecs)

In all except MCSLock, the right suggestion made when Threshold <= 10.

Suggestions that workedLength

Threshold

# of Racy Fields

Change (other) to volatile

Change to atomic array

Use synchronized block

DisBarrier 40 1 2 1LockFreeHashSet

50 1 4 1

OptimisticList

42 1 3 1

MCSLock 65 100 3 2LinearSenseBarrier.

61 10 2 1

Iterator_EBDeque

11 1 1 1

Lufact 19 1 1 1Sor 44 1 2 1Webserver Sim.

68 1 2 1

Conclusion Even experts can benefit from tool support for

detecting data races. JRF can also analyze synchronization idioms that do

not use locking. Has become an official extension of Java Path Finder http://babelfish.arc.nasa.gov/trac/jpf

JRF-E makes working suggestions for most of the data races in our experiments.

JRF-E can teach programmers the intricacies of Java Memory Model.

Thank You

Questions?