This document provides an overview of deadlocks, including: 1. What a deadlock is, using the example of two trains approaching each other. 2. The four conditions required for deadlock and examples of resource allocation graphs. 3. Methods for handling deadlocks including prevention, avoidance, detection/correction, and ignorance. 4. Details on deadlock prevention, avoidance, and algorithms like the banker's algorithm.

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Welcome
to
our presentation
on
Deadlock

2. Presented by:
Md. Sohag
Em@il : sohag.0315@gmail.com
Presented to:
Deadlock
2
8/13/2017

3. What is Deadlock ?
 A set of blocked processes each holding a resource and waiting to
acquire a resource held by another process in the set.
 Example
2 train approaches each other at crossing, both will come to full
stop and neither shall start until other has gone.
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4. Deadlock Example
 Traffic only in one direction.
 Each section of a bridge can be viewed as a resource.
 If a deadlock occurs, it can be resolved if one car backs up .
 Several cars may have to be backed up if a deadlock occurs. 8/13/2017
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5. DEADLOCK CHARACTERIZATION
 Deadlock can arise if four conditions hold simultaneously.
 Mutual exclusion
 Hold and wait
 No preemption
 Circular wait
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6. RESOURCE-ALLOCATION GRAPH
 Deadlocks can be described in term of directed graph- SYSTEM RESOURCE-
ALLOCATION GRAPH.
 Process
 Resource Type with 4 instances
 Pi requests instance of Rj
 Pi is holding an instance of Rj
p
Pi
Rj
Pi
Rj
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7. EXAMPLE OF RESOURCE-ALLOCATION GRAPH
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p3
p1 p2
R2
R1 R3
R4

8. RESOURCE-ALLOCATIONGRAPH AND A DEADLOCK
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9. METHODS FOR HANDLINGDEADLOCK
 We can deal with DL problem in 3-ways:
 Prevention/Avoidance
 Detection/Correction
 Ignorance
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10. DEADLOCK PREVENTION
 Restrain the ways request can be made.
 Mutual Exclusion – not required for sharable resources;
must hold for non-sharable resources.
 Hold and Wait – must guarantee that whenever a process
requests a resource, it does not hold any other resources.
 No Preemption –
 Circular Wait –
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11. DEADLOCK AVOIDANCE
 Requires additional information about how resources are to
be used.
 The deadlock-avoidance algorithm dynamically examines
the resource-allocation state to ensure that there can never
be a circular-wait condition.
 Resource-allocation state is defined by the number of
available and allocated resources, and the maximum
demands of the processes.
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12. SAFE, UNSAFE DEADLOCK STATE
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13. SAFE & UNSAFE STATE - Example
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14. DEADLOCK AVOIDANCE ALGORITHM
 Single instance of a resource type.
Use a resource-allocation graph
Cycles are necessary are sufficient for deadlock
 Multiple instances of a resource type.
Use the banker’s algorithm
Cycles are necessary, but not sufficient for deadlock
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15. RESOURCE ALLOCATION GRAPHALGORITHM
 Claim edge Pi  Rj indicates that process Pj may request resource
Rj; represented by a dashed line.
 Claim edge converts to request edge when a process requests a
resource.
 Request edge converted to an assignment edge when the
resource is allocated to the process.
 When a resource is released by a process, assignment edge
reconverts to a claim edge.
 Resources must be claimed a priori in the system.
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16. BANKER’S ALGORITHM
 RAG is not applicable for multiple instance of resource
 Bankers’ algorithm - Multiple instances.
 Each process claims maximum resource needs a priori.
 When a process requests a resource it may have to wait.
 When a process gets all of its resources it must return them
in a finite amount of time.
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