1. Checking nonblocking concurrent queue program
Jun Miao
York University
5/9/2018

2. Concurrent Queue Queue is a widely used ADT in computer science
Insertions and deletions follow the first-in first-out scheme

3. Node Structure Node <V> V item;
AtomicStampedReference<Node<V>> next;
AtomicStampedReference
ReferenceIntegerPair<V>:
private final V reference
private final int integer

4. Dummy Node <null, null>
Initialization
Node<E> dummy = new Node<E>(null,null);
this.head = new AtomicStampedReference<Node<E>>(dummy,0);
this.tail = new AtomicStampedReference<Node<E>>(dummy,0);
head
tail
Stamp = 0
Reference
Dummy Node <null, null>
Stamp = 0
Reference

5. Enqueue If NO CompareAndSet is used in 3 steps
Create a new node
Store current Tail and last node
Is current Tail unchanged?
Is the stored node still the last one?
Insert the node at the end
Update reference of Tail to new node
If NO
CompareAndSet is used in 3 steps
Swing Tail to the last node

6. Dequeue Is current Head unchanged?
Store current Head, Tail, and the first available node
Is the queue empty?
Get information from the first available node
Update Head
Is current Head unchanged?
Are Head and Tail pointing to the same node because Tail is not updated?

7. Properties no uncaught exceptions no data races
All listeners can be found in
javapathfinder-trunk\src\gov\nasa\jpf\tools

8. NullPointerException
Problem: q.head points to a null node
The only possible reason:
1. q.head != q.tail
2. q.head.next == null
But how does this happen?

9. Struggling with NullPointerException
NonNullChecker is used
First Attempt
2 threads
Thread 1
Enqueue()
Dequeue()
Thread 2

10. Simple is good Second Attempt Start from 1 thread with 1 operation
2 threads: 2 enqueue , 2 dequeue, 1 enqueue and 1 dequeue
3 threads: 3 enqueue ….
4 threads: 4 enqueue…
Result:

11. PreciseRaceDetector Two threads Three threads… Four thread… Result:
Enqueue|Dequeue
Enqueue|Enqueue
Dequeue|Dequeue
Three threads…
Four thread…
Result:

12. Exceptions and Data races
No Exceptions in the concurrent queue operations
No Data races
My question: is my testing enough?

13. Something interesting
This algorithm came out of a discussion with Franck van Breugel and Sergey Kulikov from the University of York. All credits for it goes to Franck and Sergey, all the bugs are mine.
Author: Willem Visser

14. Post Condition Start from q.head Print value of each node sequentially
Reach q.tail

15. Conclusion Testing the program step by step
The more enqueue operation, the more time consumed by JPF checking
Testing time increases significantly

16. Question Time