This document discusses deadlocks in operating systems. It defines a deadlock as a set of blocked processes that are each holding a resource and waiting for a resource held by another process. Four conditions must be met for a deadlock to occur: mutual exclusion, hold and wait, no preemption, and circular wait. Deadlocks can be modeled using directed resource allocation graphs. Methods for handling deadlocks include prevention, avoidance, detection, and recovery.

1. DEADLOCKS IN
OPERATING SYSTEM

2. OPERATING SYSTEM
Presented by :
Wajiha Siddiqui
Sara Ali
Aisha Hameed

3. 1. The Deadlock Problem
2. Deadlock Characterization
3. Deadlock Modeling
4. Resource Allocation Graph
5. Methods for Handling Deadlocks
 Deadlock Prevention
 Deadlock Avoidance
 Deadlock Detection
 Recovery from Deadlock

4. WHAT ARE DEADLOCKS???
A set of blocked processes each
holding a resource and waiting to
acquire a resource held by another
process in the set.
o Example
 Process-1 requests the printer, gets it
 Process-2 requests the tape unit, gets it Process-1 and
 Process-1 requests the tape unit, waits Process-2 are
 Process-2 requests the printer, waits deadlocked!

5. FOUR CONDITIONS FOR DEADLOCKS???
1. Mutual Exclusion Condition
2. Hold & Wait Condition
3. No Preemption Condition
4. Circular Wait Condition

6. DEADLOCK MODELING???
Modeled with directed graphs

7. HOW DEADLOCKS OCCUR?

8. HOW DEADLOCKS CAN BE AVOIDED???

9. RESOURCE ALLOCATION GRAPHS???
 CIRCLES
 represents processes
 SQUARE
 represents classes of identical resources
 SMALL SQUARES DRAWN INSIDE LARGE SQUARES
 Indicate separate identical resources of each class

10.  If the resource allocation graph contains no cycles then there is
no deadlock in the system at that instance.
 If there is a cycle
o If there is only one instance per resource type than there is a deadlock
o If there is more than once instance for some resource type than there may
or may not be a deadlock

11. EXAMPLE OF A RESOURCE ALLOCATION GRAPH

12. HOW TO HANDLE DEADLOCKS???
• Four major areas of interest in deadlock research
- Deadlock Prevention
- Deadlock Avoidance
- Deadlock Detection
- Deadlock Recovery

13. DEADLOCK PREVENTION???
• Prevention eliminates one of four conditions
1. Elimination Of Mutual Exclusion
2. Elimination Of Hold & Wait Condition
3. Elimination Of No Preemption Condition
4. Elimination Of Circular Wait Condition

14. DEADLOCK AVOIDANCE???
• Maximum requirements of
each resource must be stated
in advance by each process.
• Two Approaches
• Two Algorithms
o Resource Allocation Algorithms
o The Banker’s Algorithm

15. DEADLOCK DETECTION???
• Determines if deadlock has occurred
• Identifies those processes & resources
involved in the deadlock
• Algorithms can incur significant
runtime overhead

16. RECOVERY FROM DEADLOCKS???
• Process Termination
o Abort all deadlocked processes.
o Abort one process at a time until
the deadlock cycle is eliminated.
• Resource Preemption
o Selecting a victim
o Rollback
o Starvation