Chapter 3
Submitted by :-ANOOP RANA
BCA 3rd Yr
Deadlocks
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Deadlocks: Understanding and Prevention, Assignments of Operating Systems
An overview of deadlocks in computer systems, including why they occur, dealing with them, and preventing deadlocks. It includes explanations of resources, their types, and the sequence of events required to use them. The document also covers the formal definition of deadlock and the four conditions that lead to deadlock. A resource allocation graph is used to model resource allocation and identify deadlocks.
What you will learn
- Why do deadlocks occur in computer systems?
- What is a resource allocation graph and how is it used to identify deadlocks?
- What are the two types of resources in computer systems?
- How can deadlocks be prevented?
- What are the four conditions for deadlock?
Typology: Assignments
2019/2020
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Chapter 3
Submitted by :-ANOOP RANA
BCA 3rd^ Yr
Deadlocks
Chapter 3 2 Overview Resources Why do deadlocks occur? Dealing with deadlocks Ignoring them: ostrich algorithm Detecting & recovering from deadlock Avoiding deadlock Preventing deadlock
Chapter 3 4 When do deadlocks happen? (^) Suppose (^) Process 1 holds resource A and requests resource B (^) Process 2 holds B and requests A (^) Both can be blocked, with neither able to proceed (^) Deadlocks occur when … (^) Processes are granted exclusive access to devices or software constructs (resources) (^) Each deadlocked process needs a resource held by
A
B
B
A
Process 1 (^) Process 2 DEADLOCK!
CS 1550, cs.pitt.edu Chapter 3 5 (originaly modified by Ethan Using resources Sequence of events required to use a resource
Request the resource
Use the resource
Release the resource
Can’t use the resource if request is denied
Requesting process has options
(^) Block and wait for resource (^) Continue (if possible) without it: may be able to use an alternate resource (^) Process fails with error code
Some of these may be able to prevent deadlock…
CS 1550, cs.pitt.edu Chapter 3 7 (originaly modified by Ethan Four conditions for deadlock
Mutual exclusion
(^) Each resource is assigned to at most one process
Hold and wait
(^) A process holding resources can request more resources
No preemption
(^) Previously granted resources cannot be forcibly taken away
Circular wait
(^) There must be a circular chain of 2 or more processes where each is waiting for a resource held by the next member of the chain
CS 1550, cs.pitt.edu Chapter 3 8 (originaly modified by Ethan Resource allocation graphs R (^) Resource allocation modeled by directed graphs (^) Example 1: (^) Resource R assigned to process A (^) Example 2: (^) Process B is requesting / waiting for resource S (^) Example 3: (^) Process C holds T, waiting for U (^) Process D holds U, waiting for T (^) C and D are in deadlock! A S B U T C D
Graph With A Cycle But No Deadlock
Basic Facts If graph contains no cycles no deadlock If graph contains a cycle if only one instance per resource type, then deadlock (^) necessary and sufficient condition if several instances per resource type, possibility of deadlock (^) necessary condition