The document discusses the ABA problem that can occur in non-blocking concurrent queue algorithms. It shows an example of how the ABA problem can allow a thread to incorrectly retrieve another thread's data from the queue. It then describes two common solutions to the ABA problem - adding version numbers to data structures or using load-linked/store-conditional instructions. Finally, it shows pseudocode for an enqueue operation of a non-blocking concurrent queue that avoids the ABA problem by using a double-compare-and-swap with version numbers.

1. ABA Problem
Dr. C.V. Suresh Babu

2. Introduction
• Queues are everywhere in parallel applications
and operating systems
• Many researchers have proposed queues
–
–
–
–
Hwang and Briggs
Gottlieb
Massalin
Et al. etc…
• Queue performance can be critical to operating
system performance
– Scheduling Queues
– Free memory lists
– Many other critical kernel operations

3. Concurrent FIFO Queue algorithms
• Blocking algorithms risk performance
degradation
– A process can be delayed or halted at inopportune
moments
• Scheduling preemption
• Page faults
• Cache misses
– Slow processes can prevent faster ones from
completing indefinitely
• Non-Blocking algorithms must solve the ABA
problem
– During contention, some process will complete
within a given number of operations

4. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
value = SM
newVal = value +1
Stack Data = d
CAS(&SM, value, newVal)
break
Stack
SM
5
…
x
time
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X

5. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
newVal = value +1
Stack Data = d
CAS(&SM, value, newVal)
break
Stack
SM
4
…
x
time

6. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
CAS(&SM, value, newVal)
break
Stack
SM
4
…
x
time

7. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Push(z)
Stack Data = d
CAS(&SM, value, newVal)
break
Stack
SM
4
…
x
time

8. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
Stack
SM
4
…
x
time

9. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
newVal=5
Stack
SM
4
…
x
time

10. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
loop
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
newVal=5
Stack
CAS(&SM,value,newVal)
SM
5
…
z
time

11. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
loop
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
newVal=5
Stack
CAS(&SM,value,newVal)
CAS(&SM,value,newVal)
SM
5
…
z
time

12. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
loop
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
newVal=5
Stack
CAS(&SM,value,newVal)
CAS(&SM,value,newVal)
v1=x
SM
4
…
z
time

13. ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
CAS(&SM, value, newVal)
v1=Pop()
value = 5
Push (d) {
newVal = 4
data=X
return data
value = 5
newVal = 4
break
v2=Pop()
data=X
loop
CAS(&SM,value,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
CAS(&SM, value, newVal)
value = 4
break
newVal=5
Stack
CAS(&SM,value,newVal)
CAS(&SM,value,newVal)
v1=x
SM
4
…
z
CAS should fail but it succeeds
time
Thread1 has Thread2’s data

14. Solutions for ABA problem
Cache Kernel
• Add version # to data structures
• Increment # during every CAS instruction
LL/SC
• Fail if Cache Line has been written to

15. Solution for ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
DCAS(&SM, value,
v1=Pop()
value = 5
return data
Push (d) {
newVal = 4
data=X
break
value = 5
newVal = 4
<ver++,newVal>)
v2=Pop()
data=X
loop
DCAS(&SM,value,ver,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
DCAS(&SM, value,
value = 4
<ver++,newVal>)
newVal=5
Stack
break
DCAS(&SM,value,ver,newVal)
DCAS(&SM,value,ver,newVal)
Will not incorrectly succeed
SM
5
…
z
(ver != ver+2)
time

16. Solution for ABA problem
Pop () {
loop
value = SM
newVal = value -1
THREAD1
THREAD2
data = Stack Data
DCAS(&SM, value,
v1=Pop()
value = 5
return data
Push (d) {
newVal = 4
data=Z
break
value = 5
newVal = 4
<ver++,newVal>)
v2=Pop()
data=X
loop
DCAS(&SM,value,ver,newVal)
value = SM
v2 = x
newVal = value +1
Stack Data = d
Push(z)
DCAS(&SM, value,
value = 4
<ver++,newVal>)
newVal=5
Stack
break
DCAS(&SM,value,ver,newVal)
DCAS(&SM,value,ver,newVal)
Will not incorrectly succeed
SM
4
(ver != ver+2)
…
time
V1 = Z

17. Correctness Properties
1.
2.
3.
4.
5.
The linked list is always connected
Nodes only inserted after the last node
Nodes only deleted from beginning
Head always points to the first node
Tail always points to a node in the list

18. Queue # 1
• Non-Blocking Concurrent Queue
– enqueue()

19. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
node–>next.ptr = NULL
loop
tail = Q–>Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node

20. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node

21. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node

22. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
[NULL]

23. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
[NULL]

24. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
“enqueue(myQueue, D1)”
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
[NULL]

25. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
[NULL]

26. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]

27. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]
[NULL]

28. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]
tail
[NULL]

29. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]
tail
next
[NULL]

30. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]
tail
next
[NULL]

31. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS( &tail.ptr–>next, next,
<node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
[NULL]
tail
next
[NULL]

32. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
+1
[NULL]
tail
next

33. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
+1
[NULL]
tail
next

34. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
enqueue(myQueue, D1)
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
+1
if tail == Q–>Tail
if next.ptr == NULL
if CAS(&tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
endif
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
endif
endif
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
D1
+1
[NULL]

35. Concurrent enqueues
• Suppose two processes call enqueue() at
the same time

36. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS( &tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
Process 1
endif
Enqueue(myQueue, ABC)
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
ABC
endif
endif
[NULL]
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
tail
next
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
D1
[NULL]
Process 2
Enqueue(myQueue, XYZ)
XYZ
[NULL]
tail
next

37. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS( &tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
Process 1
endif
Enqueue(myQueue, ABC)
else
CAS(&Q–>Tail, tail, <next.ptr,
tail.count+1>)
ABC
endif
endif
[NULL]
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
tail
next
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
D1
+1
XYZ
[NULL]
Process 2
Enqueue(myQueue, XYZ)
tail

38. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
enqueue(Q: pointer to queue t, value: data type)
node = new node()
node–>value = value
Head
node–>next.ptr = NULL
loop
tail = Q–>Tail
Tail
next = tail.ptr–>next
if tail == Q–>Tail
if next.ptr == NULL
if CAS( &tail.ptr–>next,
next, <node, next.count+1>)
myQueue
break
Process 1
endif
Enqueue(myQueue, ABC)
else
CAS( &Q–>Tail, tail,
<next.ptr, tail.count+1>)
ABC
endif
endif
[NULL]
endloop
CAS(&Q–>Tail, tail, <node, tail.count+1>)
tail
next
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
D1
+1
XYZ
[NULL]
Process 2
Enqueue(myQueue, XYZ)
tail

39. Queue #1 (cont.)
• Non-Blocking Concurrent Queue
– dequeue()

40. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
“dequeue(myQueue, pvalue)”
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head,
head, <next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]

41. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head,
head, <next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

42. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head,
head, <next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

43. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head,
head, <next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

44. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head, head,
<next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

45. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
+1
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head, head,
<next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

46. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
+1
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head, head,
<next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

47. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
+1
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head, head,
<next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]
head
tail
next

48. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
dequeue(myQueue, pvalue)
dequeue(Q: ptr to queue t, pvalue: ptr to data
type):bool
loop
Head
head = Q–>Head
+1
tail = Q–>Tail
next = head–>next
if head == Q–>Head
Tail
if head.ptr == tail.ptr
if next.ptr == NULL
return FALSE
endif
myQueue
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head, head,
<next.ptr, head.count+1>)
break
endif
endif
endif
endloop
free(head.ptr)
D1
[NULL]

49. Concurrent dequeues
• Suppose two processes call dequeue() at
the same time

50. struct pointer_t {
node_t * ptr
uint count
}
struct node_t {
data_type value
pointer_t next
}
struct queue_t {
pointer_t Head
pointer_t Tail
}
dequeue(Q: ptr to queue t, pvalue: ptr to data type):bool
loop
head = Q–>Head
Head
tail = Q–>Tail
next = head–>next
if head == Q–>Head
if head.ptr == tail.ptr
Tail
if next.ptr == NULL
return FALSE
endif
CAS(&Q–>Tail, tail, <next.ptr, tail.count+1>)
else
# Read value before CAS, otherwise another
# dequeue might free the next node
*pvalue = next.ptr–>value
if CAS ( &Q–>Head,head, <next.ptr, head.count+1>)
break
endif
“dequeue(myQueue,
endif
endif
endloop
free(head.ptr)
return TRUE
D1
[NULL]
pvalue)”

51. Queue #2
• Two-lock Concurrent Queue

52. struct node_t {
data_type value
node_t * next
}
struct queue_t {
pointer_t Head
pointer_t Tail
lock_type H_lock
lock_type T_lock
}
initialize(Q: pointer to queue t)
node = new node()
node–>next.ptr = NULL
Q–>Head = Q–>Tail = node
Q–>H lock = Q–>T lock = FREE
dequeue(Q: pointer to queue t, pvalue: pointer to data type): boolean
lock(&Q–>H lock)
node = Q–>Head
new head = node–>next
if new head == NULL
enqueue(Q: pointer to queue t, value: data type)
unlock(&Q–>H lock)
node = new node()
return FALSE
node–>value = value
endif
node–>next.ptr = NULL
*pvalue = new head–>value
lock(&Q–>T lock)
Q–>Head = new head
Q–>Tail–>next = node
unlock(&Q–>H lock)
Q–>Tail = node
free(node)
unlock(&Q–>T lock)
return TRUE
• Algorithms have same general structure only different
data types
• No loops, ‘busy waiting’ instead
• Only dequeues access Head Lock
• Only enqueues access Tail Lock

53. Performance Parameters
• Net execution time for one million
enqueue/dequeue pairs
• 12-processor Silicon Graphics Challenge
multiprocessor
• Algorithms compiled with using highest
optimization level
• Including many hand optimizations

54. Dedicated multiprocessor
Multiprogrammed system with 3
processes per processor
Multiprogrammed system with 2
processes per processor

55. Conclusion
• NBS clear winner for multiprocessor
multiprogrammed systems
• Above 5 processors, use the new nonblocking queue
• If hardware only supports test-and-set use
two lock queue
• For two or less processors use a single
lock algorithm for queues