1. Chapter 6 Concurrency: Deadlock and Starvation I
Conditions for Deadlock
Deadlock Prevention
Deadlock Avoidance
Banker’s Algorithm
Deadlock Detection
Strategies once Deadlock Detected
Deadlock Detection Algorithm
UNIX Concurrency Mechanisms

2. Deadlock
Deadlock
Deadlock can be defined as the permanent blocking of a set of processes that either compete for system resources or communicate with each other
All deadlocks Involve conflicting needs for resources by two or more processes

3. Example of Deadlock (1) Process P Process Q ...... ...... Get A Get B
Get A Get B
Get B Get A
Release A Release B
Release B Release A

4. Progress of processes-Example of Deadlock (2)
of Q 1 2
Release A
P and Q
want A A
Required
Release B
Get A 3
deadlock
inevitable
P and Q
want B B
Required 5
Get B 4 6
Progress
of P
Get A
Get B
Release A
Release B A
Required
B Required

5. Execution Paths -Example of Deadlock (3)
1. Q acquires B and then A, and then releases B and A. When P resumes execution, it will be able to acquire both resources.
2. Q acquires B and then A. P executes and blocks on a request for A. Q releases B and A. When P resumes execution, it will be able to acquire both resources.
5. P acquires A and then B. Q executes and blocks on a request for B. P releases A and B. When Q resumes execution, it will be able to acquire both resources.
6. P acquires A and then B, and then releases A and B. When Q resumes execution, it will be able to acquire both resources.

6. Deadlock Paths -Example of Deadlock (4)
3. Q acquires B and then P acquires A. Deadlock is inevitable, because as execution process Q will block on A and P will block on B.
4. P acquires A and then Q acquires B. Deadlock is inevitable, because as execution process Q will block on A and P will block on B.

7. Example of No Deadlock (1)
Process P Process Q
Get A Get B
Release A Get A
Get B Release B
Release B Release A

8. Example of No Deadlock (2)
Progress
of Q 1 2 3
Release A 4
P and Q
want A A
Required
Release B
Get A
P and Q
want B B
Required 5
Get B 6
Progress
of P
Get A
Release A
Get B
Release B
A Required
B Required

9. Question 1 - Exercise/Home Work (1)
For Figure of No Deadlock, provide a narrative description of each of the six depicted paths.

10. Reusable Resources
A reusable resource is one that can be safely used by one process at a time and not depleted by that use
Processes obtain resources that they later release for reuse by other processes
Examples of reusable resources: processor time, I/O channels, main and secondary memory, files, databases, and semaphores
Deadlock occurs if each process holds one resource and requests the other

11. Example of Deadlock
Space is available for allocation of 200K bytes, and the following sequence of events occur
Deadlock occurs if both processes progress to their second request P1 P2
. . .
. . .
Request 80K bytes;
Request 70K bytes;
. . .
. . .
Request 60K bytes;
Request 80K bytes;

12. Consumable Resources
A consumable resource is one that can be created (produced) and destroyed (consumed) by a process
Examples of consumable resources: interrupts, signals, messages, and information in I/O buffers
Deadlock may occur if a Receive message is blocking
May take a rare combination of events to cause deadlock

13. Example of Deadlock Deadlock occurs if receive is blocking
P1 is blocked for waiting message from P2
P2 is blocked for waiting message from P1 P1 P2
. . .
. . .
Receive(P2);
Receive(P1);
. . .
. . .
Send(P2);
Send(P1);

14. Conditions for Deadlock
Three conditions of policy must be present for a deadlock to be possible:
Mutual exclusion
only one process may use a resource at a time
Hold-and-wait
a process may hold allocated resources while awaiting assignment of others
No preemption
no resource can be forcibly removed from a process holding it

15. Conditions for Deadlock
Circular wait
a closed chain of processes exists, such that each process holds at least one resource needed by the next process in the chain

16. Circular Wait Resource A Requests Held by Process P1 Process P2

17. Deadlock Prevention (1)
Mutual Exclusion
cannot be disallowed
Hold-and-Wait
The hold-and-wait condition can be prevented by:
require that a process request all its required resources at one time
block the process until all requests can be granted simultaneously
process may be held up for a long time waiting for all its requests
resources allocated to a process may remain unused for a long time. These resources could be used by other processes

18. Deadlock Prevention (2)
No preemption
This condition can be prevented in several ways:
if a process is denied a further request, the process must release the original resources
if a process cannot obtain a resource, the process may have to release its resources. Must have capability to restore to current state.
This approach is practical only when the state can be easily saved and restored later, such as the processor.

19. Deadlock Prevention (3)
Circular wait
The circular-wait condition can be prevented by :
define a linear ordering for resources
once a resource R is obtained, then it may subsequently request only those resources of types following R in the ordering.

20. Deadlock Avoidance (1)
Do not start a process if its demands might lead to deadlock
Do not grant an incremental resource request to a process if this allocation might lead to deadlock

21. Deadlock Avoidance (2)
Maximum resource requirement must be stated in advance
Processes under consideration must be independent; no synchronization requirements
There must be a fixed number of resources to allocate
No process may exit while holding resources

22. Banker’s Algorithm - Deadlock Avoidance (1)
Total amount of each resource in the system
Resource = ( R1, R2, , Rm )
Total amount of each resource not allocated to a process
Available = ( V1, V2, , Vm )

23. Banker’s Algorithm - Deadlock Avoidance (2)
Requirement of each process for each resource
C11 C C1m
Claim = C21 C C2m
Cki: process k
Cn1 Cn Cnm for resource j
Current allocation
A11 A A1m
Allocation = A21 A A2m
Aki: process k
An1 An Anm for resource j

24. Relationships - Banker’s Algorithm (3)
Ri = Vi + S Aki, for all i: All resources are
k= either available or allocated.
Cki <= Ri, for all k, i: No process can claim more than the total amount of resources in the system.
Aki <= Cki, for all k, i: No process is allocated more resources of any type than the process originally claimed to need.

25. Deadlock Avoidance Policy - Banker’s Algorithm (4)
Ri >= C(n+1)i + Sum Cki, for all i
k=1
A new process Pn+1 is only started if the maximum claim of all current processes plus those of the new process can be met.

26. Safe State -Banker’s Algorithm (5)
The state of the system is simply the current allocation of resources to processes. Thus the state consists of the two vectors, Resource and Available, and two matrices, Claim and Allocation.
Safe State
A safe state is a state in which there is at least one sequence in which all of the processes can be run to completion that does not result in a deadlock.

27. Determination of a Safe State-Banker’s Algorithm (6)
R1 R2 R R1 R2 R R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(a) Initial State - A Safe State?

28. Determination of a Safe State-Banker’s Algorithm (7)
R1 R2 R R1 R2 R R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(b) P2 Runs to Completion

29. Determination of a Safe State-Banker’s Algorithm (8)
R1 R2 R R1 R2 R3 R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(c) P1 Runs to Completion

30. Determination of a Safe State-Banker’s Algorithm (9)
R1 R2 R R1 R2 R3 R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(d) P3 Runs to Completion

31. Banker’s Algorithm - Deadlock Avoidance (10)
.....
while ((possible) and (rest != null)) do
find a Pk in rest such that
claim[k, *] - allocation[K, *] <= currentavail;
if found then
currentavail:=currentavail + allocation[K, *];
rest:= rest - {Pk}
else
possible:=false
end
end;
......

32. Determination of an Unsafe State-Banker’s Algorithm (11)
R1 R2 R R1 R2 R3 R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(a) Initial State - A Safe State?

33. Determination of an Unsafe State-Banker’s Algorithm (12)
R1 R2 R R1 R2 R3 R1 R2 R3
P P
P P Resource Vector
P P
P P R1 R2 R3
Claim Matrix Allocation Matrix Available Vector
(b) P1 Requests One unit each of R1 and R3
- An Unsafe State

34. Question 2 - Exercise/Home Work (1)
R1 R2 R3 R4 R1 R2 R3 R4 R1 R2 R3 R4 P P P P P
Claim Matrix Allocation Matrix Still Needs
Resource Vector Available Vector

35. Question 2- Exercise/Home Work (2)
Consider the system given:
a. Compute what each process still might request and display in the columns labeled “still needs.”
b. Is this system currently in a safe or unsafe state? Why?
c. Is this system currently deadlocked? Why or Why not?
d. Which processes, if any, are or may become deadlocked?
e. If a request from P3 arrives for (0, 1, 3, 4), can that request to be safely granted immediately? In what state (deadlocked, safe, unsafe) would immediately granting that whole request leave the system? Which processes, if any, are or may become deadlocked if this whole request is granted immediately?