Deadlocks Two - Computer Operating Systems - Lecture Slides, Slides of Operating Systems

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Lecture 14

Chapter 7: Deadlocks (cont)

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Chapter 7: Deadlocks

• The Deadlock Problem
• System Model
• Deadlock Characterization
• Methods for Handling Deadlocks
• Deadlock Prevention
• Deadlock Avoidance
• Deadlock Detection
• Recovery from Deadlock

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The Deadlock Problem

• A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set
• Example
  • System has 2 disk drives
  • P 1 and P 2 each hold one disk drive and each needs another one
• Example
  • semaphores A and B , initialized to 1 P 0 P 1 wait (A); wait(B) wait (B); wait(A) Docsity.com

Bridge Crossing 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 - preempt resources and rollback Docsity.com

Deadlock Characterization

• Mutual exclusion: only one process at a time can use a resource
• Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes
• No preemption: a resource can be released only voluntarily by the process holding it, after that process has

Deadlock can arise if four conditions hold simultaneously.

necessary^ conditions

sufficient^ condition

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Resource-Allocation Graph

• V is partitioned into two types:
  • P = { P 1 , P 2 , …, Pn }, the set consisting of all the processes in the system
  • R = { R 1 , R 2 , …, R (^) m }, the set consisting of all resource types in the system
• request edge – directed edge P 1 → R (^) j
• assignment edge – directed edge R → P

A set of vertices V and a set of edges E.

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Example of a Resource Allocation

Graph

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Basic Facts

• If graph contains no cycles ⇒ no deadlock
• If graph contains a cycle ⇒
  • if only one instance per resource type, then deadlock
  • if several instances per resource type, possibility of deadlock

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Methods for Handling Deadlocks

• Ensure that the system will never enter a deadlock state - deadlock prevention - design decision - deadlock avoidance - runtime decision
• Allow the system to enter a deadlock state and then recover - deadlock recovery - periodic runtime decision Docsity.com

Deadlock Prevention

• Mutual Exclusion – not required for sharable resources; - must hold for nonsharable resources - not possible - must always be supported by the O/S
• Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources - Require process to request and be allocated all its resources before it begins execution,

Restrain the ways request can be made

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Deadlock Avoidance

• Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need
• The deadlock-avoidance algorithm dynamically examines the resource- allocation state to ensure that there can never be a circular-wait condition - mainly a scheduling problem

Requires that the system has some additional a priori information available

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Safe State

• When a process requests an available resource, system must decide - if immediate allocation leaves the system in a safe state
• System is in safe state if there exists a sequence < P 1 , P 2 , …, P (^) n > of ALL the processes is the systems such that - for each Pi , the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj , with j < I
• That is: Docsity.com

Avoidance algorithms

• Single instance of a resource type
  • Use a resource-allocation graph
• Multiple instances of a resource type
  • Use the banker’s algorithm

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Resource-Allocation Graph Scheme

• Claim edge Pi → R (^) j indicated that process Pj may request resource R (^) j ; 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 Docsity.com