ch 24 deadlocks

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Ceng 334 - Operating Systems 2.4-1

Chapter 2.4 : Deadlocks

Process concept

Process scheduling

Interprocess communication

Deadlocks

Threads


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What is Deadlock?

Process Deadlock

A process is deadlocked when it is

waiting on an event which will neverhappen

System Deadlock

A system is deadlocked when one ormore processes are deadlocked


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Necessary Conditions for a

Deadlock

Mutual Exclusion

Shared resources are used in a

mutually exclusive manner

Hold & Wait

Processes hold onto resources theyalready have while waiting for theallocation of other resources


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Necessary Conditions for a

Deadlock (Cont.)

No PreemptionResources can not be preempted

until the process releases them

Circular WaitA circular chain of processes exists

in which each process holdsresources wanted by the next processin the chain


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No Deadlock Situation

If you canprevent at least one

of the necessary deadlockconditionsthen you wont

have a DEADLOCK


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The Ostrich Algorithm

Pretend there is no 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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Ways of Handling Deadlock

Deadlock Prevention

Deadlock Detection Deadlock Avoidance

Deadlock Recovery


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Deadlock Prevention

Remove the possibility of deadlock

occurring by denying one of the four

necessary conditions:

Mutual Exclusion (Can we share everything?)

Hold & Wait

No preemptionCircular Wait


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Implementation

A process is given its resources on a

"ALL or NONE" basis

Either a process gets ALL its required

resources and proceeds or it getsNONE of them and waits until it can

Denying the Hold & Wait


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Advantages

It works

Reasonably easy to code

Problems

Resource wastage

Possibility of starvation


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Denying No preemption

Implementation

When a process is refused a resource

request, it MUST release all other

resources it holds

Resources can be removed from aprocess before it is finished with

them


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Advantages

It worksPossibly better resource utilisation

Problems

The cost of removing a process'sresources

The process is likely to lose work it

has done. (How often does thisoccur?)

Possibility of starvation


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Denying Circular Wait

Implementation

Resources are uniquely numbered

Processes can only request resources

in linear ascending order

Thus preventing the circular waitfrom occurring


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Advantages

It worksHas been implemented in some OSes

Problems

Resources must be requested in ascending

order of resource number rather than asneeded

Resource numbering must be maintainedby someone and must reflect every addition

to the OS

Difficult to sit down and write just writecode


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Deadlock Avoidance

Allow the chance of deadlock occur

But avoid it happening..

Check whether the next state (change in

system) may end up in a deadlock situation


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Bankers Problem

Suppose total bank capital is 1000 MTL Current cash : 1000- (410+210) = 380 MTL

Customer

c1

c2

Max. Need

800

600

Present Loan

410

210

Claim

390

390


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Dijkstra's Banker's Algorithm

Definitions

Each process has a LOAN, CLAIM,

MAXIMUM NEED

LOAN: current number of resources held

MAXIMUM NEED: total number resources

needed to complete

CLAIM: = (MAXIMUM - LOAN)


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Assumptions

Establish a LOAN ceiling (MAXIMUM

NEED) for each process

MAXIMUM NEED < total number of resourcesavailable (ie., capital)

Total loans for a process must be less than

or equal to MAXIMUM NEED Loaned resources must be returned back in

finite time


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Algorithm

1. Search for a process with a claim that cansatisfied using the current number of

remaining resources (ie., tentatively grant

the claim)2. If such a process is found then assume that

it will return the loaned resources.

3. Update the number of remaining resources4. Repeat steps 1-3 for all processes and

mark them


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DO NOT GRANT THE CLAIM if at least

one process can not be marked.

Implementation

A resource request is only allowed ifit results in a SAFE state

The system is always maintained in a

SAFE state so eventually all requestswill be filled


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Advantages

It worksAllows jobs to proceed when a prevention

algorithm wouldn't

ProblemsRequires there to be a fixed number of

resources

What happens if a resource goes down?

Does not allow the process to change its

Maximum need while processing


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Safe and Unsafe States (1)

Demonstration that the state in (a) is safe

(a) (b) (c) (d) (e)


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Safe and Unsafe States (2)

Demonstration that the sate in (b) is not safe

(a) (b) (c) (d)


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Banker's Algorithm for Multiple Resources

Example of banker's algorithm with multiple resources


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Deadlock Detection

Methods by which the occurrence of

deadlock, the processes and resources

involved are detected.

Generally work by detecting a circular wait

The cost of detection must be considered

One method is resource allocation graphs


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Deadlock Recovery

Recover from the deadlock by

removing the offending processes

The process being removed may lose

work


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Problems

Most systems do not support the removal

and then restarting of a process.

Some processes should NOT be removed.

It is possible to have deadlock involving

tens or even hundreds of processes


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Implementation

Processes are simply killed off (lost forever)Usually some sort of priority order exists for

killing

Support for suspend/resume (rollback)

Some systems come with checkpoint/restart

featuresDevelopers indicate a series of checkpoints when

designing a software application

So a process only need be rolled back to the last

checkpoint, rather than back to the beginning


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Question : What is the simplest and

most used method to recover from adeadlock?

ch 24 deadlocks

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