1. Chapter 7 Deadlock

2. Outline Problem illustration Deadlock definition Resource types
Resource allocation graph
Deadlock conditions
Deadlock avoidance
Deadlock detection

3. Dining Philosophers Problem

4. Dining Philosophers Problem

5. First Solution

6. Second Solution

7. A Solution based on Monitor

8. A Solution based on Monitor

9. Deadlock
Permanent blocking of a set of processes that either compete for system resources or communicate with each other
Often involve conflicting needs for resources by two or more processes
No efficient solution
prevention, avoidance, detection

10. Two Resource Types
Reusable resources
Consumable resources

11. Reusable Resources
Used by only one process at a time and not depleted by that use
Processes obtain resources that they later release for reuse by other processes

12. Reusable Resources
Processors, I/O channels, main and secondary memory, devices, and data structures such as files and databases
Deadlock occurs if each process holds one resource and requests the other

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

14. Consumable Resources Created (produced) and destroyed (consumed)
Interrupts, signals, messages, and information in I/O buffers

15. Example of Deadlock Deadlock occurs if receive is blocking . . . . . .
Receive(P2);
Receive(P1);
. . .
. . .
Send(P2, M1);
Send(P1, M2);

16. Conditions for Deadlock
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

17. Conditions for Deadlock
No preemption
No resource can be forcibly removed from a process holding it
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

18. Resource Allocation Graphs
Directed graph that depicts a state of the system of resources and processes

19. Resource Allocation Graphs

20. Resource Allocation Graph for Figure (b)

21. Resource Allocation Graph for Figure (b)

22. Possibility of Deadlock
Mutual Exclusion
No preemption
Hold and wait

23. Existence of Deadlock Mutual Exclusion No preemption Hold and wait
Circular wait
Self-read Chap 7.4, to know
How to prevent deadlock?
What’s the disadvantage of using deadlock prevention?

24. Deadlock Avoidance
A decision is made dynamically on whether the current resource allocation request will, if granted, potentially lead to a deadlock
Requires knowledge of future process requests

25. Two Approaches to Deadlock Avoidance
Resource-allocation-graph algorithm (Chap 7.5.2) is only applicable to systems with single instance of each resource type
Do not grant an incremental resource request to a process if this allocation might lead to deadlock (Banker’s Algorithm)
Can be used to a banking system to allocate cash.

26. Resource Allocation Denial
Referred to as the banker’s algorithm
State of the system is the current allocation of resources to processes
Safe state is where there is at least one sequence that does not result in deadlock
Unsafe state is where any sequence will result in deadlock

27. Determination of a Safe State
Is there a resource allocation sequence that does not result in deadlock

28. Determination of a Safe State

29. Determination of a Safe State

30. Determination of a Safe State

31. Safe to Grant P1’s Request?
P1 requests one unit each of R1 and R3

32. Safe to Grant P1’s Request?

33. Determination of an Unsafe State

34. Deadlock Avoidance Logic

35. Deadlock Avoidance Logic

36. In-Class Exercise
Consider a system consisting of four processes and a single resource. The current state of the claim and allocation matrices are
Claim Matrix = [3, 2, 9, 7]T
Allocation Matrix = [1, 1, 3, 2]T
Resource Vector = [?]
What is the minimum number of units of the resource needed to be available for this state to be safe?

37. C = [3, 2, 9, 7]T
A = [1, 1, 3, 2]T
RV = [?], AV[?-7]
C = [3, 2, 9, 7]T
A = [1, 1, 3, 2]T
RV = [7] AV = [0]
X Cannot make progress on any process!  need at least 1 extra instance of resource

38. C = [3, 2, 9, 7]T
A = [1, 1, 3, 2]T
RV = [8] AV = [1]
Finish P2:
C = [3, 0, 9, 7]T
A = [1, 0, 3, 2]T
AV = [2]
Finish P1:
C = [0, 0, 9, 7]T
A = [0, 0, 3, 2]T
AV = [3]
X Cannot make progress on any process!
 need at least 2 extra instances of resource

39. C = [0, 0, 9, 7]T
A = [0, 0, 3, 2]T
RV = [10] AV = [5]
Finish P4:
C = [0, 0, 9, 0]T
A = [0, 0, 3, 0]T
AV = [7]
Finish P3:
C = [0, 0, 0, 0]T
A = [0, 0, 0, 0]T
AV = [10]

40. Deadlock Avoidance
Maximum resource requirement must be stated in advance
Processes under consideration must be independent; their execution order must be unconstrained by any synchronization requirements
There must be a fixed number of resources to allocate
No process may exit while holding resources

41. Deadlock Detection
Available vector

42. Deadlock Detection Algorithm
Mark each process that has a row in the allocation matrix of all zeros
Initialize a temporary vector W to equal to available vector

43. Deadlock Detection Algorithm
Find an index i such that process i is currently unmarked and the ith row of request matrix is less than or equal to W. If no such row is found, terminate the algorithm and all unmarked processes are those involved in the deadlock.
If such a row is found, mark process i and add the corresponding row of the allocation matrix to W.

44. Strategies Once Deadlock Detected
Abort all deadlocked processes
Back up each deadlocked process to some previously defined checkpoint, and restart all processes
Original deadlock may occur

45. Strategies Once Deadlock Detected
Successively abort deadlocked processes until deadlock no longer exists
Successively preempt resources until deadlock no longer exists

46. Advantages and Disadvantages