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The Deadlock Problem

System Model

Deadlock Characterization

Methods for Handling Deadlocks

Deadlock Prevention

Deadlock Avoidance

Deadlock Detection

Recovery from Deadlock

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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.

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Suppose system has 2 tape drives.

P1 and P2 each hold one tape drive and each needs

another one.

Deadlock

Occurs

Process 1 Process 2

Tape Drive 1 Tape Drive 2

holds holds

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I need quad

A and B

I need quad

B and C

I need quad

C and B

I need quad

D and A

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HALT until B

is free

HALT until C

is free

HALT until D

is free

HALT until A

is free

Deadlock Occurs

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Deadlock can be defined formally as follows:

A set of processes is deadlocked if

each process in the set is

waiting for an event that only

another process in the set can

cause

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8

A process must request a resource before using it,

and must release the resource after using it.

A process may request as many resources as itrequires to carry out its designated task

The number of resources requested may not

exceed the total number of resources available in

the system

E.g a process cannot request three printers if the system

only has two)

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9

Conditionsfor Resource

Deadlocks

Mutualexclusion

condition

Hold and

wait

condition

No

preemption

condition.

Circular wait

condition.

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10

Only one process can have exclusive control of a shared

resource and if any other process request that veryparticular resource then it has to wait till the time the

resource is released

Mutualexclusioncondition

Hold andwait

condition

A process holding one resource and waiting to acquire

another resource that are currently held by other process

1st Condition

2nd Condition

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A circular chain of processes exist

in which each process holds

number of resources which are

requested by the next process in

the chain

Resources cannot be preempted; A resource can be

released only voluntarily by the process holding it, after

that process has completed its task.

Nopreemption

condition.

Circular waitcondition.

3rd Condition

4rth Condition

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V is partitioned into two types:

P= {P1, P2, , Pn}, the set consisting of

all the processes in the system.

R = {R1, R2, , Rm}, the set consisting ofall resource types in the system.

request edgedirected edge P1 Rj

assignment edgedirected edge Rj Pi

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Four conditions can be modeled by using directed graphs

The graphs have two type of nodes

Process (as circle)

Resources (as square)

Requesting a resource

Holding a resource

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P1 is holding an instance

of resource R2

P1 is waiting for an

instance of a resource R1


of resource R1 & R2


instance of a resource R3


of resource R3


of resource R3


instance of resource R2

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RESOURCE

ALLOCATION

GRAPH WITH A

CYCLE BUT NO

DEADLOCK

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no deadlock No cycles

cycles

if only one instance

per resource type,

then deadlock

if several instances

per resource type,possibility of deadlock

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no deadlock No cycles

cycles

if only one instance

per resource type,

then deadlock

if several instances

per resource type,possibility of deadlock

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20

1st Strategy

2nd Strategy

4th Strategy

3rd Strategy

Just ignore the problem and pretend deadlock

never happened

Prevention, negating one of the four required

conditions

Dynamic avoidance by careful resourceallocation

Detection and recovery. Let deadlocks occur,detect them, take action.

Strategies for dealing with deadlocks:

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Just ignore the problem

Reasonable if

deadlocks occur very rarely

cost of prevention is high

UNIX and Windows takes this approach

It is a trade off between

convenience

correctness

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Deadlock prevention is the process of making it logically

impossible for one of the 4 deadlock conditions to hold.

Mutual exclusion

Hold and Wait

No preemption

Circular wait

Restrain the ways

resource allocation

requests can be made to

insure that at least one of

the four necessaryconditions is violated.

2nd strategy:

Deadlock

Prevention

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not required for sharable resources;

must hold for non sharable resources.

Restrain the ways request can be made.

2nd strategy:

Deadlock

Prevention

Must be supported by the OS

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2nd strategy:

Deadlock

Prevention

To ensure that the hold-and-wait neveroccurs in the system, we must guaranteethat, whenever a process requests aresource, it does not hold any otherresources.

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2nd strategy:

Deadlock

Prevention

Require a process to request

and be allocated all its

resources before it beginsexecution

Allow a process to request

resources only when the

process has none.

2 Ways

1

2

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Problems

No useful work possible with some resources while waiting for

others Also ties up resources other processes could be using

May not know required resources at start of run

Process could starve

A process that needs several popular resources may haveto wait indefinitely, because at least one of the resourcesthat it needs is always allocated to some other process

2nd strategy:

Deadlock

Prevention

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2nd strategy:

Deadlock

Prevention

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Preempted resources are added to the

list of resources for which the process iswaiting.

Process will be restarted only when it

can regain its old resources, as well as

the new ones that it is requesting.

2nd strategy:

Deadlock

Prevention

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This approach to the deadlock problemanticipates deadlock before it actually occurs. Thisapproach employs an algorithm to access thepossibility that deadlock could occur and actingaccordingly. This method differs from deadlockprevention, which guarantees that deadlockcannot occur by denying one of the necessaryconditions of deadlock.

If the necessary conditions for a deadlock are inplace, it is still possible to avoid deadlock by beingcareful when resources are allocated

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30

2nd strategy:

Deadlock

Avoidance

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Simplest and most useful model requires that each process declare themaximum number of resources of each type that it may need.

Avoid deadlock by acquiring additional information on how the

resources are to be requested.

For Example:

Process A uses resource 1 and 2 and then

releases them

Process B uses resource 2 and then 1.

3rd

strategy:

If system have this

information before handthan prior to resource

allocation a deadlock

avoidance algorithm can

determine that which

process should wait so

that a deadlock can beavoided in future

2nd strategy:

Deadlock

Avoidance

3rd

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When a process requests an available resource, system must decide ifimmediate allocation leaves the system in a safe state.

System is in safe state if there exists a safe sequence of all processes.

Sequence is safe if for each Pi, the resources that Pi canstill request can be satisfied by currently available resources

If Pi resource needs are not immediately available, then Pican

wait until all Pjhave finished. When Pj is finished, Pi can obtain needed resources, execute,

return allocated resources, and terminate.

When Piterminates, Pi+1 can obtain its needed resources, and soon.

3

strategy:

Deadlock

Avoidance

2nd strategy:

Deadlock

Avoidance

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Avoidance ensure that a system will never enter an unsafe state.

If a system is in

safe state no

deadlocks

If a system is in

unsafe state

possibility of

deadlock.

2nd strategy:

Deadlock

Avoidance

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A state is known as SAFE state, if the system can allocate resources

to each process in SOME order and still avoid deadlock

A system is in Safe State only if there exists a Safe Sequence.

1

15

2

07

3

110

For example: Suppose

system with 10 resources

and following allocation

4

7

9

2nd strategy:

Deadlock

Avoidance

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Bankers Algorithm Made by Dijkstra

Examines if an allocation leads to a safe or unsafe state

Each process must a priori claim 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.

2nd strategy:

Deadlock

Avoidance

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2nd strategy:

Deadlock

Avoidance

Available:

Vector of length m. If available [j] = k, there are kinstances of resource type Rjavailable.

Max:

n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type

Rj.

Allocation

n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj.

Need

n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task

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2nd strategy:

Deadlock

Avoidance

Need[i,j]= Max[i,j]Allocation [i,j].

Need Matrix can be calculated as

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Allocation Max Available

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 2 0 0 3 2 2

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

2nd strategy:

Deadlock

Avoidance

Snapshot at time T0:

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The content of the matrix. Need is defined to be

Need = MaxAllocation.

Need

A B C

P0 7 4 3

P1 1 2 2

P2 6 0 0

P3 0 1 1

P4 4 3 1

2nd strategy:

Deadlock

Avoidance

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The system is in a safe state since the sequence

< P1, P3, P4, P2, P0> satisfies safety criteria.

2nd strategy:

Deadlock

Avoidance

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Check that Request Available (that is, (1,0,2) (3,3,2) true.

Allocation Need Available

A B C A B C A B C

P0 0 1 0 7 4 3 2 3 0

P1 3 0 2 0 2 0

P2 3 0 1 6 0 0

P3 2 1 1 0 1 1

P4 0 0 2 4 3 1

Executing safety algorithm shows that sequence satisfies safety requirement.

Can request for (3,3,0) by P4 be granted?

Can request for (0,2,0) by P0 be granted?

2nd strategy:

Deadlock

Avoidance

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No limitation on Resource allocation

Use algorithm to detect possible deadlock

Resolve any deadlock found

When should algorithm be run?

When resource is claimed

Periodically, every n time unit

During idle periods

3rd strategy:

Deadlock

Detection

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Make a resource graph

Determine the wait for graph

Locate the cycles

3rd strategy:

Deadlock

Avoidance

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3rd strategy:

Deadlock

Avoidance

Resource-Allocation Graph

Corresponding wait-for graph

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E is the existing resource vector

A is the available resource vector

C is the current allocation matrixCij means number of instance of each resource class

Pi currently holds

R is the request MatrixRij means the number of instance of resources j that

Pi wants

3rd strategy:

Deadlock

Detection

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Deadlock detection algorithm:

1. Look for an unmarked process, Pi , for which the i-th row ofR is lessthan or equal toA.

2. If such a process is found, add the i-th row ofC to A, mark the

process, and go back to step 1.

3. If no such process exists, the algorithm terminates.

3rd strategy:

Deadlock

Detection

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48

Algorithm Exercise : Is

there any Deadlock inthe system?

1

23

No Deadlock

3rd strategy:

Deadlock

Detection

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Example

Process 1 request cant be fulfilled as CD Rom not available

Process 2 request cant be fulfilled as scanner is not free

Process 3 can run

A=(0 0 0 0 )

Process 3 will return its resources

A=(2 2 2 0)

Now Remaining Processes can run

3rd strategy:

Deadlock

Detection

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Recovery through preemption

take a resource from some other process

depends on nature of the resource

Recovery through rollback

checkpoint a process periodically

use this saved state

restart the process if it is found deadlocked

Recovery through killing processes

crudest but simplest way to break a deadlock

kill one of the processes in the deadlock cycle

the other processes get its resources

choose process that can be rerun from the beginning

4rth strategy:

Recovery

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