CS294-32: Dynamic Data Race Detection

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CS294-32: Dynamic Data Race Detection. Koushik Sen UC Berkeley. Race Conditions. class Ref { int i; void inc() { int t = i + 1; i = t; } } . Courtesy Cormac Flanagan. Race Conditions. class Ref { int i; void inc() { int t = i + 1; i = t; } }

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### CS294-32: Dynamic Data Race Detection

Koushik Sen

UC Berkeley

Race Conditions

class Ref {

int i;

void inc() {

int t = i + 1;

i = t;

}

}

Courtesy Cormac Flanagan

Race Conditions

class Ref {

int i;

void inc() {

int t = i + 1;

i = t;

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

• A race condition occurs if
• two threads access a shared variable at the same time without synchronization
• at least one of those accesses is a write
Race Conditions

t1

class Ref {

int i;

void inc() {

int t = i + 1;

i = t;

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

t2

RD(i)

RD(i)

WR(i)

WR(i)

Lock-Based Synchronization

class Ref {

int i; // guarded by this

void inc() {

synchronized (this) {

int t = i + 1;

i = t;

}

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

• Field guarded by a lock
• Lock acquired before accessing field
• Ensures race freedom
Dynamic Race Detection
• Happens Before [Dinning and Schonberg 1991]
• Lockset:
• Eraser [Savage et al. 1997]
• Precise Lockset [Choi et al. 2002]
• Hybrid [O'Callahan and Choi 2003]
Dynamic Race Detection
• Precise knowledge of the execution
• No False positive [Happens Before]
• Unless you try to predict data races [Lockset]
• Produce false negatives
• because they only consider a subset of possible program executions.
What we are going to analyze?
• A trace representing an actual execution of a program
• Trace is sequence of events:
• MEM(m,a,t): thread t accessed memory local m, where the access a 2 {RD,WR}
• m can be o.f, C.f, a[i]
• ACQ(l,t): thread t acquires lock l
• Ignore re-acquire of locks. l can be o.
• REL(l,t): thread t releases lock l
• SND(g,t): thread t sends message g
• If t1 calls t2.start(), then generate SND(g,t1) and RCV(g,t2)
• If t1 calls t2.join(), then generate SND(g,t2) and RCV(g,t1)
How to generate a trace?

class Ref {

int i; // guarded by this

void inc() {

synchronized (this) {

int t = i + 1;

i = t;

}

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

Instrument a Program

class Ref {

int i; // guarded by this

void inc() {

synchronized (this) {

int t = i + 1;

i = t;

}

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

Sample Trace

class Ref {

int i; // guarded by this

void inc() {

synchronized (this) {

i = i + 1;

}

}

}

Ref x = new Ref(0);

parallel {

x.inc(); // two calls happen

x.inc(); // in parallel

}

assert x.i == 2;

Sample Trace

ACQ(4365,t1);

MEM(4365.i,RD,t1)

MEM(4365.i,WR,t1)

REL(4365,t1);

ACQ(4365,t2);

MEM(4365.i,RD,t2)

MEM(4365.i,WR,t2)

REL(4365,t2);

Compute Locks Held by a Thread

L(t) = locks held by thread t. How do we compute L(t)?

Locks Held

Sample Trace

L(t1)={}, L(t2)={}

L(t1)={4365}, L(t2)={}

L(t1)={4365}, L(t2)={}

L(t1)={4365}, L(t2)={}

L(t1)={}, L(t2)={}

L(t1)={}, L(t2)={4365}

L(t1)={}, L(t2)={4365}

L(t1)={}, L(t2)={4365}

L(t1)={}, L(t2)={}

ACQ(4365,t1);

MEM(4365.i,RD,t1)

MEM(4365.i,WR,t1)

REL(4365,t1);

ACQ(4365,t2);

MEM(4365.i,RD,t2)

MEM(4365.i,WR,t2)

REL(4365,t2);

Let us now analyze a trace
• Instrument Program
• Run Program => A Trace File
• Analyze Trace File
Happens-before relation
• [Dinning and Schonberg 1991]
• Idea: Infer a happens-before relation Á between events in a trace
• We say e1Á e2
• If e1 and e2 are events from the same thread and e1 appears before e2 in the trace
• If e1 = SND(g,t) and e2 = RCV(g,t’)
• If there is a e’ such that e1Á e’ and e’ Á e2
• REL(l,t) and ACQ(g,t’) generates SND(g,t) and RCV(g,t’)
• We say e1 and e2 are in race, if
• e1 and e2 are not related by Á,
• e1 and e2 are from different threads
• e1 and e2 access the same memory location and one of the accesses is a write
Happens-before: example 1

x := x + 1

ACQ(mutex)

v := v + 1

REL(mutex)

Any two accesses of shared variables are in the relation happens-before

ACQ(mutex)

v := v + 1

REL(mutex)

x := x + 1

Happens-before: example 2

ACQ(mutex)

v := v + 1

REL(mutex)

x := x + 1

Therefore, only this second execution reveals the existing datarace!!

x := x + 1

ACQ(mutex)

v := v + 1

REL(mutex)

Eraser Lockset
• Savage,Burrows,Nelson,Sobalvarro,Anderson
• Assume a database D storing tuples

(m,L)

where:

• m is a memory location
• L is a set of locks that protectm
• Initially D contains a tuple (m,U) for each memory location m, where U is the universal set
How it works?
• For an event MEM(m,a,t) generate the tuple (m,L(t))
• Let (m, L’) be the tuple present in D
• Report race over memory location m if L(t) Å L’ = empty set
• Replace (m, L’) by (m, L(t) Å L’) in D

L(t2) = {mutex}

(v,{mutex}) 2D

L(t1)={mutex}

(v,{mutex}) 2D

Eraser: Example 1

ACQ(mutex)

v := v + 1

REL(mutex)

ACQ(mutex)

v := v + 1

REL(mutex)

L(t2) = {mutex2}

(v,{}) 2D

L(t1) = {mutex1}

(v,{mutex1}) 2D

Eraser: Example 2

ACQ(mutex1)

v := v + 1

REL(mutex1)

ACQ(mutex2)

v := v + 1

REL(mutex2)

Warning!!

Lockset

Track lockset

Shared-exclusive

Track lockset

race condition!

Local

second

Track lockset

Shared-exclusive

Track lockset

race condition!

Local

second

second

Track lockset

Shared-exclusive

Track lockset

Shared

race condition!

T1(L1,L2)

v

• false alarm

T2(L2,L3)

T3(L1,L3)

Eraser: Problem

ACQ(L2,L3)

ACQ(L3,L1)

ACQ(L1,L2)

v := v + 1

v := v + 1

v := v + 1

REL(L3,L2)

REL(L1,L3)

REL(L2,L1)

Precise Lockset
• Choi, Lee, Loginov, O'Callahan, Sarkar, Sridharan
• Assume a database D storing tuples

(m,t,L,a)

where:

• m is a memory location
• t is a thread accessing m
• L is a set of locks held by t while accessingm
• a is the type of access (read or write)
• Initially D is empty
How it works?
• For an event MEM(m,a,t) generate the tuple (m,a,L(t),t)
• If there is a tuple (m’,a’,L’,t’) in D such that
• m = m’,
• (a = WR) Ç (a’=WR)
• L(t) Å L’ = empty set
• t  t’
• Report race over memory location m
Optimizations
• Stop adding tuples on m once a race on m is detected
• Do not add (m,a,L,t) to D if (m,a,L’,t) is already in D and L’ µ L
• Many more …
Precise Lockset

x := x + 1

ACQ(mutex)

ACQ(mutex)

v := v + 1

v := v + 1

REL(mutex)

REL(mutex)

x := x + 1

D

(x, RD,{},t1)

(v,RD,{mutex},t2)

(x,WR,{},t1)

(v,WR,{mutex},t2)

(v,RD,{mutex},t1)

Conflict

detected!

(x,RD,{},t2)

(v,WR,{mutex},t1)

(x,WR,{},t2)

Precise Lockset

ACQ(m2,m3)

ACQ(m3,m1)

ACQ(m1,m2)

v := v + 1

v := v + 1

v := v + 1

REL(m3,m2)

REL(m1,m3)

REL(m2,m1)

D

(v,RD,{m1,m2},t1)

(v,RD,{m2,m3},t2)

(v,RD,{m1,m3},t3)

(v,WR,{m1,m2},t1)

(v,WR,{m2,m3},t2)

(v,WR,{m1,m3},t3)

No conflicts detected!

Precise Lockset: Not so Precise

x := x + 1

t2.start()

Precise Lockset gives Warning: But no warning with Happens-Before

x := x + 1

Hybrid Dynamic Data Race Detection
• Relax Happens Before
• No happens before relation between REL(l,t) and a subsequent ACQ(l,t)
• Maintain Precise Lockset along with Relaxed Happens-Before
Hybrid
• O'Callahan and Choi
• Assume a database D storing tuples

(m,t,L,a,e)

where:

• m is a memory location
• t is a thread accessing m
• L is a set of locks held by t while accessingm
• a is the type of access (read or write)
• e is the event associated with the access
• Initially D is empty
How it works?
• For an event e = MEM(m,a,t) generate the tuple (m,a,L(t),t,e)
• If there is a tuple (m’,a’,L’,t’,e’) in D such that
• m = m’,
• (a = WR) Ç (a’=WR)
• L(t) Å L’ = empty set
• t  t’
• e and e’ are not related by the happens before relation, i.e., :(e Á e’) Æ:(e’ Á e)
• Report race over memory location m
Hybrid Dynamic Data Race Detection

x := x + 1

Precise Lockset detects a data race, but e2 Á e3. Therefore, hybrid technique gives no warning

t2.start()

x := x + 1

D

(x,RD,{},t1,e1)

(x,RD,{},t2,e3)

(x,WR,{},t1,e2)

(x,WR,{},t2,e4)

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Distributed Computation
• A set of processes: {p1,p2,…,pn}
• No shared memory
• Communication through messages
• Each process executes a sequence of events
• send(m) : sends a message with content m
• Internal event: changes local state of a process
• ith event from process pj is denoted by eij

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Distributed Computation as a Partial Order
• Distributed Computation defines a partial order on the events
• e ! e’
• e and e’ are events from the same process and e executes before e’
• e is the send of a message and e’ is the receive of the same message
• there is a e’’ such that e ! e’’ and e’’ ! e’

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Distributed Computation as a Partial Order
• Problem: An external process or observer wants to infer the partial order or the computation for debugging
• No global clock
• At each event a process can send a message to the observer to inform about the event
• Message delay is unbounded

Observer

Can we infer the partial order?
• From the observation:
• Can we associate a suitable value with every event such that
• V(e) < V(e’) , e ! e’
• We need the notion of clock (logical)

e12 e13 e11 e21 e23 e43 e33 e31 e32 e22

Lamport’s Logical Time
• All processes use a counter (clock) with initial value of zero
• The counter is incremented by and assigned to each event, as its timestamp
• A send (message) event carries its timestamp
• For a receive (message) event the counter is updated by
• Send the counter value along with an event to the observer

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Example

1

2

3

4

5

1

6

2

1

3

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Example
• Problem with Lamport’s logical clock:
• e ! e’ ) C(e) < C(e’)
• C(e) < C(e’) ) e ! e’

X

1

2

3

4

5

1

6

2

1

3

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Example
• Problem with Lamport’s logical clock:
• e ! e’ ) C(e) < C(e’)
• C(e) < C(e’) ) e ! e’

X

1

2

3

4

5

1

6

2

1

3

Vector Clock
• Vector Clock: Process ! Nat
• V: P ! N
• Associate a vector clock Vp with every process p
• Update vector clock with every event as follows:
• Internal event at p_i:
• Vp(p) := Vp(p) + 1
• Send Message from p:
• Vp(p) := Vp(p) + 1
• Send Vp with message
• Receive message m at p:
• Vp(i) := max(Vp(i),Vm(i)) for all i 2 P, where Vm is the vector clock sent with the message m
• Vp(p) := Vp(p) + 1

p3

e13

e23

e33

e43

m1

m4

m2

e22

e32

e12

p2

m3

p1

e11

e21

e31

Physical Time

Example

V = (a,b,c) means V(p1)=a, V(p2)=b, and V(p3)=c

(0,0,1)

(0,1,2)

(2,1,3)

(2,1,4)

(2,2,4)

(0,1,0)

(2,3,4)

(1,0,0)

(2,0,0)

(3,0,0)

Intuitive Meaning of a Vector Clock
• If Vp = (a,b,c) after some event then
• p is affected by the ath event from p1
• p is affected by the bth event from p2
• p is affected by the cth event from p3
Comparing Vector Clocks
• V · V’ iff for all p 2 P, V(p) · V’(p)
• V = V’ iff for all p 2 P, V(p) = V’(p)
• V < V’ iff V · V’ and V  V’
• Theorem: Ve < Ve’ iff e ! e’
• Send an event along with its vector clock to the observer
Definition of Data Race
• Traditional Definition (Netzer and Miller 1992)

x=1

if (x==1) …

Definition of Data Race
• Traditional Definition (Netzer and Miller 1992)

x=1

X

send(m)

if (x==1) …

Operational Definition of Data Race
• We say that the execution of two statements are in race if they could be executed by different threads temporally next to each other and both access the same memory location and at least one of the accesses is a write

x=1

x=1

X

send(m)

if (x==1) …

if (x==1) …

Temporally next

to each other

Race Directed Random Testing: RACEFUZZER
• RaceFuzzer: Race directed random testing
• STEP1: Use an existing technique to find set of pairs of state transitions that could potentially race
• We use hybrid dynamic race detection
• Static race detection can also be used
• Transitions are approximated using program statements
Race Directed Random Testing: RACEFUZZER
• RaceFuzzer: Race directed random testing
• STEP1: Use an existing technique to find set of pairs of state transitions that could potentially race
• We use hybrid dynamic race detection
• Static race detection can also be used
• Transitions are approximated using program statements
• STEP2: Bias a random scheduler so that two transitions under race can be executed temporally next to each other
RACEFUZZER using an example

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Run ERASER: Statement pair (s5,s6) are in race

RACEFUZZER using an example

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Run ERASER: Statement pair (s5,s6) are in race

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Goal: Create a trace exhibiting the race

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Example Trace:

s1: g1();

s2: g2();

s3: g3();

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: o1.f = 1;

s6: if (o1.f==1)

s7: ERROR;

s4: g4();

s5: o2.f = 1;

Racing Statements

Goal: Create a trace exhibiting the race

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1();

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s6: if (o1.f==1)

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s6: if (o1.f==1)

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s6: if (o1.f==1)

Postponed = { }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s6: if (o1.f==1)

Do not postpone

Postponed = {}

s6: if (o1.f==1)

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s5: o2.f = 1;

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s5: o2.f = 1;

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s5: o2.f = 1;

Race?

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s5: o2.f = 1;

Race?

NO

o1.f ≠ o2.f

Postponed = {s6: if (o1.f==1) }

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s5: o2.f = 1;

Postponed = {s6: if (o1.f==1), }

s5: o2.f = 1;

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

Postponed = {s6: if (o1.f==1), s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

Postponed = {s6: if (o1.f==1), s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

Postponed = {s6: if (o1.f==1), s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

Postponed = {s6: if (o1.f==1), s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

Race?

YES

o1.f = o1.f

Postponed = {s6: if (o1.f==1), s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s6: if (o1.f==1) s5: o1.f = 1;

Postponed = {s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

s6: if (o1.f==1)

Postponed = {s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

s6: if (o1.f==1)

Racing Statements

Postponed = {s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

s6: if (o1.f==1)

s7: ERROR;

Racing Statements

Postponed = {s5: o2.f = 1;}

RACEFUZZER using an example

(s5,s6) in race

foo(o1);

sync foo(C x) {

s1: g1()

s2: g2();

s3: g3();

s4: g4();

s5: x.f = 1;

}

bar(o1);

bar(C y) {

s6: if (y.f==1)

s7: ERROR;

}

foo(o2);

Execution:

s1: g1();

s1: g1();

s2: g2();

s2: g2();

s3: g3();

s3: g3();

s4: g4();

s4: g4();

s5: o1.f = 1;

s6: if (o1.f==1)

s7: ERROR;

s5: o2.f = 1;

Racing Statements

Postponed = { }

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Race

Racing Pair: (8,10)

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Racing Pair: (8,10) Postponed Set = {Thread2}

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Another Example

1: lock(L);

2: f1();

3: f2();

4: f3();

5: f4();

6: f5();

7: unlock(L);

8: if (x==0)

9: ERROR;

}

10: x = 1;

11: lock(L);

12: f6();

13: unlock(L);

}

Hit error with 0.5 probability

Implementation
• RaceFuzzer: Part of CalFuzzer tool suite
• Instrument using SOOT compiler framework
• Instrumentations are used to “hijack” the scheduler
• Implement a custom scheduler
• Run one thread at a time
• Use semaphores to control threads
• Because we cannot instrument native method calls

lock(L1);

X=1;

unlock(L1);

lock(L2);

Y=2;

unlock(L2);

Implementation
• RaceFuzzer: Part of CalFuzzer tool suite
• Instrument using SOOT compiler framework
• Instrumentations are used to “hijack” the scheduler
• Implement a custom scheduler
• Run one thread at a time
• Use semaphores to control threads
• Because we cannot instrument native method calls

ins_lock(L1);

lock(L1);

ins_write(&X);

X=1;

unlock(L1);

ins_unlock(L1);

ins_lock(L1);

lock(L2);

Y=2;

unlock(L2);

ins_unlock(L1);

Custom

Scheduler

RACEFUZZER: Useful Features
• Classify real races from false alarms
• Inexpensive replay of a concurrent execution exhibiting a real race
• Separate some harmful races from benign races
• No false warning
• Very efficient
• We instrument at most two memory access statements and all synchronization statements
• Embarrassingly parallel
RACEFUZZER: Limitations
• Not complete: can miss a real race
• Can only detect races that happen on the given test suite on some schedule
• May not be able to separate all real races from false warnings
• Being random in nature
• May not be able to separate harmful races from benign races
• If a harmful race does not cause in a program crash
• Each test run is sequential
Summary
• Claim: testing (a.k.a verification in industry) is the most practical way to find software bugs
• We need to make software testing systematic and rigorous
• Random testing works amazingly well in practice
• Randomizing a scheduler is more effective than randomizing inputs
• We need to make random testing smarter and closer to verification
• Bias random testing
• Prioritize random testing
• Find interesting preemption points in the programs
• Randomly preempt threads at these interesting points
java.lang.StringBuffer

/**

... used by the compiler to implement the binary string concatenation operator ...

String buffers are safe for use by multiple threads. The methods are synchronized so that all the operations on any particular instance behave as if they occur in some serial order that is consistent with the order of the method calls made by each of the individual threads involved.

*/

/*# atomic */ public class StringBuffer { ... }

use of stale len may yield StringIndexOutOfBoundsException

inside getChars(...)

java.lang.StringBuffer

public class StringBuffer {

private int count;

public synchronized int length() { return count; }

public synchronized void getChars(...) { ... }

/*# atomic */

public synchronized void append(StringBuffer sb){

int len = sb.length();

...

...

...

sb.getChars(...,len,...);

...

}

}

java.lang.StringBuffer

public class StringBuffer {

private int count;

public synchronized int length() { return count; }

public synchronized void getChars(...) { ... }

/*# atomic */

public synchronized void append(StringBuffer sb){

int len = sb.length();

...

...

...

sb.getChars(...,len,...);

...

}

}

StringBuffer.append is not atomic:

Start:

at StringBuffer.append(StringBuffer.java:701)

Commit: Lock Release

at StringBuffer.length(StringBuffer.java:221)

at StringBuffer.append(StringBuffer.java:702)

Error: Lock Acquire

at StringBuffer.getChars(StringBuffer.java:245)

at StringBuffer.append(StringBuffer.java:710)