1. Deadlock Prevention & Avoidance
CS 314 Operating Systems
Deadlock Prevention & Avoidance
Department of Computer Science
Southern Illinois University Edwardsville
Spring, 2019
Dr. Hiroshi Fujinoki
Deadlock_Detection/000

2. CS 314 Operating Systems Deadlock Prevention
Try to prevent any one of the four necessary conditions for a deadlock
 Mutual Exclusion
 Hold & Wait
 Circular wait
 Non preemptive resources
Deadlock_Avoidance/001

3. CS 314 Operating Systems Attacking mutual exclusion
Mutual Exclusion = While a process is using a resource no other process
can use the same resource
Problem
Some resources inherently require mutual exclusion
Example
Printers
Files for updates
CD-Rs
Deadlock_Avoidance/002

4. CS 314 Operating Systems Attacking hold & wait
 Require all processes to acquire all the needed resources at once
(a) Resource request does not have to be at the beginning of a process
(b) If any of the required resource is not available, drop everything
(then try it again later)
(c) Any process can not make a request for any new resource, while
the process currently holds some resource
 A process does NOT have to request all the resources at once
(a) Whenever an additional resource is needed, drop all what
are possessed by this process and request everything at once
(b) If any process steals your resource(s) when you temporarily
have to wait until all resources become available
Deadlock_Avoidance/003

5. CS 314 Operating Systems Attacking hold & wait Possible “Hold & Wait”
Time
# of resources acquired
1-(a)
1-(c)
# of resources acquired
Time
“Hold & Wait”
1-(b)
Deadlock_Avoidance/004

6. missing, drop everything
CS 314 Operating Systems
Attacking hold & wait
Then you try to
get all you need
# of resources acquired
2-(a)
Drop all what
you possess first
Time
# of resources acquired
Time
2-(b)
Another
Process
If any of resources is
missing, drop everything
Deadlock_Avoidance/005

7. CS 314 Operating Systems Attacking non-preemption
= Allow preemption
= You process can loose your resource at any time
Problem
The same problem we had for “mutual exclusion”
No one will be happy in loosing a CD-R drive in the middle
of burning a CD
Deadlock_Avoidance/006

8. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Request
(X, Y, Z)
(Y, Z) A B
Deadlock_Avoidance/007

9. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned
(X, Y, Z) A B
Request
(Y, Z)
Deadlock_Avoidance/008

10. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned
(X, Y, Z) A B
Assigned
(Y, Z)
Request
Deadlock_Avoidance/009

11. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned
Deadlock A B
Assigned
Requested
(X, Y, Z)
(Y, Z)
Requested
Assigned
Deadlock_Avoidance/010

12. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned
Process B blocks
Assigned
Request
(X, Y, Z) A B
(Y, Z)
Request
Deadlock_Avoidance/011

13. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned
Process B blocks
Process A
Completes A B
Assigned
Request
(X, Y, Z)
(Y, Z)
Assigned
Deadlock_Avoidance/012

14. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Request B
(Y, Z)
Deadlock_Avoidance/013

15. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Assigned B
(Y, Z)
Requested
Deadlock_Avoidance/014

16. CS 314 Operating Systems Circular-Wait
Assign unique ID for every non-sharable resource in a system
Every process must start requesting resources in either ascending
or descending order of resource IDs X1 Y2 Z3
Process B
completes
Assigned B
(Y, Z)
Assigned
Deadlock_Avoidance/015

17. CS 314 Operating Systems Deadlock Avoidance
When a process makes a request to a resource, the OS simulates if it will cause
a deadlock in the future if no one releases their currently-held resource:
If it will cause a deadlock
If it will not cause a deadlock
the request is denied (“unsafe state”)
the requested resource is granted (“safe state”)
Deadlock_Avoidance/016

18. CS 314 Operating Systems The two states defined in Deadlock Avoidance
 Safe State
 Unsafe State
A system is in “safe state” if there is at least one assignment schedule
to complete all the processes without deadlock and without any process
releasing their resources
(deadlock will never happen even in the worst case)
If a system is in safe state, deadlock can always be avoided
If a system is not in safe state, the system must be in “unsafe state”
(deadlock can happen in the worst case)
If a system is in unsafe state, deadlock may not be avoided
Deadlock_Avoidance/016

19. CS 314 Operating Systems Requests Pending Process Status Process
Assigned
MAX. Need B A C 2 3 4 9 7
The number of requests a process
made, but not granted because OS
does not have the requested resources
If a process has at least one
pending request, process is BLOCKED
If a process gets all required resources
then, the process becomes COMPLTED
Otherwise a process is RUNNING
Process
Assigned
MAX. Need
Requests Pending
Process Status B A C 2 3 4 9 7
RUNNING
Deadlock_Avoidance/017

20. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
Deadlock_Avoidance/018-A

21. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
But Not Deadlock yet
If Process A requests 4 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 3 4 9 7 4
BLOCKED
RUNNING 4 2
RUNNING 2
RUNNING
Deadlock_Avoidance/018-B

22. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Deadlock = all (3) processes blocked
waiting for additional resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
Unsafe state = no one can complete
if no one releases resource
If Process A requests 4 more instances of this resource
This is “unsafe state”
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 7 3 4 9 7 4
RUNNING
BLOCKED 4 2
RUNNING 2
RUNNING
But Not Deadlock yet
Deadlock_Avoidance/018-C

23. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
If Process A requests 1 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 7 4 9 7 1
RUNNING
BLOCKED 2
RUNNING 2
RUNNING
Deadlock_Avoidance/019

24. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
If Process B requests 2 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 7 4 9 7 1
BLOCKED 2 2
BLOCKED
RUNNING 2
RUNNING
Deadlock_Avoidance/020

25. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
Now Deadlock
If Process C requests 2 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 7 4 9 7 1
BLOCKED 2 2
BLOCKED 2 2
RUNNING
BLOCKED
Deadlock_Avoidance/021

26. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
This is “safe state”
If Process B requests 2 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 3 4 9 7
RUNNING 2 4 4 2 2
RUNNING 2
RUNNING
Deadlock_Avoidance/022

27. CS 314 Operating Systems Assume: 11 instances of the resource R exist
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 2 3 4 9 7
RUNNING 4
RUNNING
RUNNING
This is “safe state”
If Process B requests 2 more instances of this resource
Process
Assigned
MAX. Need
Requests Pending
Process Status
Available B A C 3 9
RUNNING 6
COMPLETED
RUNNING 2 7
RUNNING
Deadlock_Avoidance/023

28. Process
Assigned
MAX. Need
Requests Pending
Process Status B A C 2 4 7 14 5
RUNNING
Available 6