Deadlocks: Prevention, Detection, and Recovery, Slides of Computer Science

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Deadlocks

Chapter 7

Chapter Objectives

• To develop a description of deadlocks, which

prevent sets of concurrent processes from

completing their tasks

• To present a number of different methods for

preventing or avoiding deadlocks in a

computer system.

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).

• Several cars may have to be backed up if a deadlock occurs.
• Starvation is possible.

System Model

• Resource types R 1 , R 2 ,.. ., R m

CPU cycles, memory space, I/O devices

• Each resource type R i has W i instances.

• Each process utilizes a resource as follows:

• request
• use
• release

Resource-Allocation Graph

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

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

Resource-Allocation Graph (Cont.)

• Process
• Resource Type with 4 instances
• P (^) i requests instance of Rj
• P (^) i is holding an instance of R (^) j

Pi

Pi Rj

Rj

Resource Allocation Graph With A

Deadlock

Graph With A Cycle But No Deadlock

Methods for Handling Deadlocks

• Ensure that the system will never enter a

deadlock state.

• Allow the system to enter a deadlock state

and then recover.

• Ignore the problem and pretend that

deadlocks never occur in the system; used by

most operating systems, including UNIX.

Deadlock Prevention

Restrain the ways request can be made.

Make sure at least one of the four conditions for deadlock

cannot hold:

• 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. - Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. - Low resource utilization; starvation possible.

Deadlock Avoidance

• Requires that the system has some additional a priori

information available.

• 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.
• Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes.

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 in the systems such that for each P (^) i , the resources that P (^) i can still request can be satisfied by currently available resources + resources held by all the P (^) j , with j < i.
• That is:
  • If P (^) i resource needs are not immediately available, then P (^) i can wait until all P (^) j have finished.
  • When P (^) j is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.
  • When P (^) i terminates, P (^) i +1 can obtain its needed resources, and so on.

Safe, Unsafe , Deadlock State

Example

Suppose a system has 12 tape drives and 3 processes.

At time t0, the system is as follows:

Process Max need Current allocated P0 10 5 P1 4 2 P2 9 2

3 tape drives are unallocated.

Is the system safe?

Suppose process P2 is allocated another tape drive at time t1.

Is the system safe?