1. Lecture 26 Syed Mansoor Sarwar
Operating Systems
Lecture 26
Syed Mansoor Sarwar

2. © Copyright Virtual University of Pakistan
Agenda for Today
Review of previous lecture
Monitors—Dining Philosophers
The deadlock problem
Deadlock characterization
Deadlock handling
Deadlock prevention
Recap of lecture
19 April 2019
© Copyright Virtual University of Pakistan

3. © Copyright Virtual University of Pakistan
Review of Lecture 25
Dining philosophers problem
High-level language constructs
Critical region
Monitor
19 April 2019
© Copyright Virtual University of Pakistan

4. Dining Philosophers Example
monitor dp {
enum {thinking, hungry, eating} state[5];
condition self[5];
void pickup(int i) // Following slides
void putdown(int i) // Following slides
void test(int i) // Following slides
void init() {
for (int i = 0; i < 5; i++)
state[i] = thinking; }
19 April 2019
© Copyright Virtual University of Pakistan

5. © Copyright Virtual University of Pakistan
Dining Philosophers
void pickup(int i) {
state[i] = hungry;
test(i);
if (state[i] != eating)
self[i].wait(); }
19 April 2019
© Copyright Virtual University of Pakistan

6. © Copyright Virtual University of Pakistan
Dining Philosophers
void putdown(int i) {
state[i] = thinking;
// test left and right // neighbors
test((i+4) % 5);
test((i+1) % 5); }
19 April 2019
© Copyright Virtual University of Pakistan

7. © Copyright Virtual University of Pakistan
Dining Philosophers
void test(int i) {
if ((state[(i+4)%5]!= eating)
&& (state[i] == hungry) &&
(state[(i+1)%5]!= eating)) {
state[i] = eating;
self[i].signal(); }
19 April 2019
© Copyright Virtual University of Pakistan

8. © Copyright Virtual University of Pakistan
The Deadlock Problem
A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set.
19 April 2019
© Copyright Virtual University of Pakistan

9. © Copyright Virtual University of Pakistan
The Deadlock Problem
Example
System has 2 tape drives.
P1 and P2 each hold one tape drive and each needs another one.
19 April 2019
© Copyright Virtual University of Pakistan

10. © Copyright Virtual University of Pakistan
The Deadlock Problem
Example: semaphores A and B, initialized to 1
P0 P1
wait (A); wait(B);
wait (B); wait(A);
19 April 2019
© Copyright Virtual University of Pakistan

11. © Copyright Virtual University of Pakistan
The Deadlock Problem
signal(A); P0 P1
signal(B);
19 April 2019
© Copyright Virtual University of Pakistan

12. © Copyright Virtual University of Pakistan
The Deadlock Problem
The Dining Philosophers problem … all philosophers become hungry at the same time, pick up the chopsticks on their right and wait for getting the chopsticks on their left.
19 April 2019
© Copyright Virtual University of Pakistan

13. Bridge Crossing Example
Traffic only in one direction.
Each section of a bridge can be viewed as a resource.
19 April 2019
© Copyright Virtual University of Pakistan

14. Bridge Crossing Example
If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback).
Several cars may have to be backed up if a deadlock occurs.
Starvation is possible.
19 April 2019
© Copyright Virtual University of Pakistan

15. © Copyright Virtual University of Pakistan
System Model
Resource types R1, R2, . . ., Rm (CPU cycles, memory space, I/O devices)
Each resource type Ri has Wi instances.
Each process utilizes a resource as follows:
request → use → release
19 April 2019
© Copyright Virtual University of Pakistan

16. Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.
Mutual exclusion: only one process at a time can use a resource.
19 April 2019
© Copyright Virtual University of Pakistan

17. Deadlock Characterization
Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
19 April 2019
© Copyright Virtual University of Pakistan

18. Deadlock Characterization
No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
19 April 2019
© Copyright Virtual University of Pakistan

19. Deadlock Characterization
Circular wait: There exists a set {P0, P1, …, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and P0 is waiting for a resource that is held by P0.
P0 → P1 → P2 → … → Pn → P0
19 April 2019
© Copyright Virtual University of Pakistan

20. Resource Allocation Graph
A set of vertices V and a set of edges E.
V is partitioned into two types:
P = {P1, P2, …, Pn}
R = {R1, R2, …, Rm}
E = {Request Edges, Assignment Edges}
Request Edge: P1  Rj
Assignment Edge: Rj  Pi
19 April 2019
© Copyright Virtual University of Pakistan

21. Resource Allocation Graph
Process
Resource Type with 4 instances
Pi requests instance of Rj
Pi is holding an instance of Rj Pi Pi
19 April 2019
© Copyright Virtual University of Pakistan

22. © Copyright Virtual University of Pakistan
Example Graph
19 April 2019
© Copyright Virtual University of Pakistan

23. © Copyright Virtual University of Pakistan
Graph with a Deadlock
19 April 2019
© Copyright Virtual University of Pakistan

24. Graph with a Cycle but No Deadlock
19 April 2019
© Copyright Virtual University of Pakistan

25. © Copyright Virtual University of Pakistan
Basic Facts
If graph contains no cycles  no deadlock.
If graph contains a cycle 
if only one instance per resource type, then deadlock.
if several instances per resource type, possibility of deadlock.
19 April 2019
© Copyright Virtual University of Pakistan

26. © Copyright Virtual University of Pakistan
Recap of Lecture
Monitor-based implementation of the dining philosophers problem
The deadlock problem
The system model
Deadlock characterization
The resource allocation graph
19 April 2019
© Copyright Virtual University of Pakistan

27. Lecture 26 Syed Mansoor Sarwar
Operating Systems
Lecture 26
Syed Mansoor Sarwar