Description about the DEADLOCKS in operating system as well as its handling procedures and prevention.

1. “Deadlocks” in OS
Presented By : ZAHID ALI
Ciit/FA15-BCE-021/Atk

2. 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 .
• In simple words, Deadlock occurs when a process
request the resources , the resources are not available at
that time so the process enter into waiting state . The
requesting resources are held by another waiting process
, both are in waiting state, this situation is called
Deadlock.

3. System Model
Request.
The process requests the resource.
Use.
The process can operate on the resource.
Release.
The process releases the resource.

4. Examples :
 System has two disk drives .
P1 and P2 each hold one disk drive and each
needs another one.
 Two trains approaches each other at crossing,
both will come to full stop each other and neither
shall start until other has gone.

5. Conditions for Deadlock :
Mutual Exclusion :
Processes claim the exclusive control of the resources they
require .
Hold and Wait :
Processes hold resources already allocated to them while
waiting for additional resources.
No preemption :
Resources cannot be removed from the process holding
them until the resources are used to completion.
Circular Wait :
A circular chain of processes exists such that each process
hold one or more resources that are being requested by
next process in the chain.

6. Traffic Example :
• Traffic only in one direction.
• Each section of the bridge can be viewed as a resource.
• Deadlock is occurred and it can be resolves if one car
backs up ( preempt resources and rollback).

7. Resource Allocation
Graphs
• 1) Resource Allocation with no deadlock
• 2) Resource Allocation with deadlocked.

8. Methods of Handling
Deadlocks
• We can use a protocol to prevent or avoid deadlocks,
ensuring that the system will never enter a deadlocked state.
• We can allow the system to enter a deadlocked state, detect
it, and recover.
• We can ignore the problem altogether and pretend that
deadlocks never occur in the system.

9. Deadlock Prevention
Prevention Eliminates one of the Four
Conditions
• Elimination of Mutual Exclusion
• Elimination of Hold&Wait condition
• Elimination of No preemption condition
• Elimination of Circular Wait Condition

10. Deadlock Avoidance
Dead lock avoidance technique is used
to avoid deadlock.
 Safe State: if deadlock not occur then it is safe state.
 Unsafe State: if deadlock occur then it is unsafe
state. The idea of avoiding deadlock is simply not allow
the system to enter an unsafe state.

11. Resource Allocation Graph
• Safe State:
• Unsafe State:

12. Banker’s Algorithm
Multiple instances
Each process must have a priority claim for
maximum use.
When a process requests a resource it may have
to wait.
When a process gets all its resources it must
return them in a finite amount of time.

13. Recovery from Deadlocks
• Process Termination
Abort all the process until the deadlock cycle is
eliminated.
• Resource Preemption
Preempt resources by selecting a victim and roll
it back by using starvation.

14. THANKS..!!
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