1. Lecture 18: Deadlock: Conditions, Detection and Avoidance (cont.)

2. Review: Deadlock Modeling
Modeled with directed graphs
resource R assigned to process A
process B is requesting/waiting for resource S
process C and D are in deadlock over resources T and U

3. In this lecture Strategies for dealing with Deadlocks
just ignore the problem altogether
detection and recovery
dynamic avoidance
careful resource allocation
prevention
negating one of the four necessary conditions

4. Ignore the problem

5. Deadlock Detection Suppose you were given
The resources currently held by each
The additional resources each process wants
Question: Is the system deadlocked?

6. Deadlock Detection One resource of each type
Multiple resources of each type

7. Detection with One Resource of Each Type
Note the resource ownership and requests
A cycle can be found within the graph, denoting deadlock

8. Detection with multiple Resources of Each Type
Is there some order of execution of the processes which allows all of them to complete?
If yes, then not deadlocked
If no such order exists, then deadlocked
After each process finishes, it releases resources back to the system

9. Example 1 Process Resources Held Resources Needed P1 A=1, B=1 1 of B
B=1, C=1
1 of C P3
C=1, A=1
none
Resource Type
Copies A 2 B C
Not deadlocked, since P3 can run, releasing resources for P2,
which releases resources to P1 upon finishing

10. Example 2 Process Resources Held Resources Needed P1 A=1, B=1 1 of B
B=1, C=1
1 of C P3
C=1, A=1
1 of A
Resource Type
Copies A 2 B C
Deadlocked, since nobody can make progress

11. Example 3 Process Resources Held Resources Needed P1 A=1, B=1 1 of B
B=1, C=1
2 of A P3
C=1, A=1
None
Resource Type
Copies A 2 B C
Deadlocked:
Let P3 complete. P3 releases resources. But then, nobody else can proceed.

12. Detection with Multiple Resources of Each Type (1)
Data structures needed by deadlock detection algorithm

13. Detection with Multiple Resources of Each Type (2)
Look for an unmarked process, P_i, for which the ith row of R is less than or equal to A.
If such a process is found, add the ith row of C to A, mark the process, and go back to 1
If no such process exists, the algorithm terminates.

14. Detection with Multiple Resources of Each Type (3)
An example for the deadlock detection algorithm

15. Recovery from Deadlock
Recovery through Preemption
Recovery through Rollback
Recover through Killing-process