Deadlock 6

8/14/2019 Deadlock 6

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Deadlock

CPSC 355/655

version 10/2002


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Deadlock

Deadlock exists if a process is waiting for

an event which can never occur. e.g., two processes each waiting for the other

to set a flag, or

to send a message, or

to give up a resource

Our focus: contention over resources processes waiting for requested resources

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Indefinite Postponement

where the timing is such that a process does not get to do what it is trying to do, or

does not get whatever it is trying to get,

indefinitely,

but there is a possibility that it may eventually succeed

e.g., a process stuck in the ready queue because of priority

in a busy wait, trying to enter a critical section, unable to

because of timing

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Deadlock versusIndefinite Postponement

Indefinite Postponement exampleA: while (TSET LOCK); B: while (TSET LOCK);

manipulate X; manipulate X;

LOCK FALSE; LOCK FALSE;

Deadlock example

C: begin critical section X D: begin critical section Y

begin critical section Y begin critical section Xmanipulate X & Y manipulate X & Y

end critical section Y end critical section X

end critical section X end critical section Y

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Resource Allocation Graph

RAG shows

which processes have requested which resources and are currently waiting for them to be granted

which resources are owned by which processes

i.e., which resources have been granted to whichprocesses

A I

process A requests resource I

B II

process B owns resource II10/2000


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RAG ExampleA owns I and IIB requests IV

C owns III and requests ID owns IV and requests IIIE requests I, II, and III

A

B

C

D

E

I

II

III

IV

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Directed Cycles

Process Pi owns resource Ri

and requests Ri+1(mod n)

fori = 0, 1, ..., n-1.

P0

P1

R0 R1

R2

P2

Pn-1

...

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Multiple Resources

There may be multiple resources of the same type - I.e.,

equivalent resources.

Dots in the resource box indicate how many when there are

multiple equivalent resources.

two resources

three resources

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Directed Cyclesand Multiple Resources

A directed cycle does not guarantee deadlock

if there are multiple resources of the same type.

No deadlock exists here.

C II

A

BI

What about here?

C II

A

BI

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Conditions Necessary forDeadlock

Exclusive use A process is allowed exclusive access to resources it has

been granted.

Hold and wait When a process requests a resource and is waiting for it, it

will not relinquish control over resources it already owns.

No preemption Nothing can take away resources from a process which

wishes to retain them.

Circular wait A directed cycle exists in the RAG.

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Deadlock Exists if ...

The four necessary conditions hold, Exclusive use

Hold and wait

No preemption

Circular wait

AND

there is no way to break the directed cycle byhaving a needed resource become availablewithout violating the other 3 necessary conditions.

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Cycles May Be Broken

If a process finishes using a resource andvoluntarily gives it up, then this is not aviolation of any of the necessary conditions.

Exclusive useHold and wait

No preemption

Circular wait

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There is no deadlock if ...

A series of processes

can finish

voluntarily giving up resources

which are then reassigned

eventually freeing up one in each cycle

C

DB

A

E

C

DB

A

E

DB

C

DB

A

DB

A

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Dealing with Deadlock

Three approaches Detection and Recovery

detect deadlock when it occurs

somehow break deadlock and allow most processes tocontinue useful work

Prevention

determine how resources are managed focuses on the resources owned by the requesting process and the

available resources

will not take into account the entire state of the current system

Avoidance

determine if resource can be granted based on current state of entire system

which processes own which resources10/2000


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Proactive vs ReactiveApproaches

Proactive keep deadlock from happening

Prevention

Avoidance

Reactive

act in response to deadlock Detection and Recovery

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Deadlock Detection

Does a deadlock exist now? Assume that there will be no further new requests for

resources.

Graph theory approach applied to the RAG prune off processes which have all requested resources

allocated

assume these processes have now finished

reassign their resources

repeat until

all processes are pruned, or

deadlock

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Reallocating Resources

many ways to reassign resources optimal strategy: always give to a process which willhave all its requested resources

D

C

B

A

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Detection Example 1I

D

C

BA

III

II

I

D

C

BAIII

II

I

D

C

B III

II

I

III

II

I

B III

II

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Detection Example 2

I

D

C

BA

IV

III

II

I

D

C

BA

IV

III

II

I

D

C

B

IV

III

II

I

D

C

IV

III

II

I

C

IV

III

II

I

IV

III

II

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Detection Example 3

I

D

C

B

A IIIII

IV

V

VI VIIE

F I

D

C

B

A IIIII

IV

V

VI VIIE

FI

C

B

A IIIII

IV

V

VI VIIE

FI

C

B

A IIIII

IV

V

VI VII

F

10/2000

I

C

B

A IIIII

IV

V

VI VII


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Ingredients for Detection

Processes P1, P2, ...,Pn

Resources R1, R2, ...,Rm

OWNik = the number ofRkowned by Pi

REQik = the number ofRkrequested by Pi

FREEk = the number ofRkavailable

FINi = boolean indicating whetherPi is finished

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Detection Algorithm

continue true

while (continue)find i such that FINi = false and REQik FREEk for all k

if i does not exist then

continue false

else

FREEk FREEk + OWNik for all kFINi true

end-if

end-while

if there exists i such that FINi

= false then

return DEADLOCK

else

return NO DEADLOCK

end-if

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Detection withOne Resource of Each Type

A directed cycle is sufficient. assuming that the other necessary conditions hold

But finding a cycle in a graph with N nodestakes O(N2) time.

Reduce graph to WAIT FOR graph. eliminating many nodes

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WAIT FOR Graphs

BA

BA

reduces to WAIT FOR graph

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Y

X

Z

X

Z

Y

reduces to WAIT FOR graph


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Deadlock Recovery

Choose a victim process either terminate it or

preempt its resources.

one or many in a deadlock

Process preemption take away one or all resources

it must be able to recover

starvation possible

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Which Process?

can process recover

how much time has process used

how much additional time needed to finish

which resources are being used

which resources are needed

priority

how many more processes will have to be victims

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Choosing a Victim

CBA

CBA CBA

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Deadlock Prevention

DENY one of the 4 necessary conditions

deny exclusive use

deny hold and wait

deny no preemption

deny circular wait

Choose any one of these.

Adopt rules governing resources which preventthis one condition from ever occurring.

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Deny Hold and Wait

e.g., require all resources to be

requested at the same time at job level before or when process starts

at sub-job (step) level

give up all resources at end of step

request all resources for next step

can be very wasteful can lead to low resource usage

may lead to indefinite postponement most practical in batch environment

in other environments, applicable to swap files

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Deny No Preemption

e.g., take away resources which are neededby another process

process must either be able to return resources to same state, or

roll back to an earlier state

may lead to indefinite postponement

applicable to CPU

real memory

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Deny Circular Wait

e.g., order resources and require that

they be granted in that order order resources

R0, R1, R2, R3, ...

R0 < R1 < R2 < R3 < ... require that ifRi < Rk ,

then Ri can only be requested before Rk

applicable to internal resources PCB

page tables

kernel semaphores in Linux7/2001


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Proof by Contradiction

R0 < R1 < R2 < R3 < ...

ifRi < Rk , then Ri can only be requested before Rk

so ifPm owns Rs and is requesting Rt, this impliesthat

Rs < Rt

if there is a circular waitP0 owns R and is requesting R

P1 owns R and is requesting R

...Pn-1 owns R and is requesting R

this implies

R < R < R < ... < R < R 10/2000


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Denying Circular Wait



...Pn-1 owns R and is requesting R

this implies

R < R < R < ... < R < R

Po

P1

P2

R

R

R

Pn-1

...

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Deadlock Prevention Rules

Applicability of rules is determined by which resources one process is requesting, and/or

which resources are available, and/or

which resources the process has, and/or

the type of a resource

They are applied without respect to which

resources are owned by other processes.

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Deadlock AvoidanceRequests for resources are

granted, or denied (postponed)

depending on the overall state of the system.

A resource is granted if this cannot lead to adeadlock in the future.

Knowledge about all future resource needs ofeach process is required.

A process must declare its maximum resource needs

when it starts. 10/1999


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Claim Edges

IA

IIB

A owns I

B is requesting II

IIIC C will request III

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Avoidance vs Prevention

Avoidance would deny II to B once A owns I.

Prevention might allow the following.

Assume A & B each need both I & II and A owns I.

A

B

I II

A

B

I II

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Safe States

Assume that we know each process maximum resource needs

each process will finish once its has all the resource itneeds and release all resources

The system state is Safe if available resourcescan be allocated to processes in some order sothat

each process will finish

no deadlock will occur

An Unsafe state may lead to deadlock. may not currently be deadlock

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What is Unsafe?

Granting ownership ofII to B would make thestate Unsafe.

A

B

I II

A

B

I II

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Safe Example

I

C

B

A

II

III

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I

C

B

A

II

III

I

C

B

A

II

III

I

C

B

A

II

III

I

C

B

A

II

III

I

C

B

A

II

III


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Unsafe Examples

IV

F

E

D

V

VI

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VII

K

H

G


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Bankers Algorithm

Bank

has money in several currencies currencies can not be exchanged

will loan it out

but only if available funds are sufficient

will not possibly lead to deadlock

Customers have various needs

credit line specifies the maximum need

request funds in stages

will finish when all needs are fulfilled

and return all funds to bank

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Bankers Algorithm(for 1 type of resource)

To determine if a request should be granted toprocess Pk:

ALLOCATEDi = number of resources already allocated to process PiNEED

i

= number of additional resources needed by Pi

to finish

AVAIL = number of resources available

REQUEST = current request

1 If REQUEST > AVAIL, then deny request because not enough

resources.

2 Pretend to allocate request and apply the Safety algorithm.

3 If Safety Algorithm returns SAFE, then grant request otherwisedeny it.

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Safety Algorithm


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Safety Algorithm

(for 1 type of resource)

Safety algorithm

continue true

while (continue)

find i such that FINi = false and Ni AV

if i does not exist then

continue falseelse

AV AV + AiFINi true

end-while

if there exists i such that FINi = false then

return UNSAFE

else

return SAFE

Pretend to grant resource

Ak = ALLOCk + REQUEST

Nk

= NEEDk

- REQUEST

AV = AVAIL - REQUEST

Ai = ALLOCATEDi for i k

Ni = NEEDi for i k

FINi = false for all i

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Applying Safety Algorithm #2

Capital = 10

Customer Credit Current

Line Loan

A 5 3

B 6 4

C 4 0

Cash left (AVAIL) = 3

C requests 2. Should this be granted?

Would new state be Safe?

New Still

Loan Need

3 2

4 2

2 2

Cash left = 1

UNSAFE because

all customers need

at least 2 more

No

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Applying Safety Algorithm #3

Note that a state is SAFE only if all can finish.

Capital = 10

Customer Credit Current

Line Loan

A 2 1

B 6 3

C 9 3

Cash left (AVAIL) = 3

C requests 2. Should this be granted?

Would new state be Safe?

New Still

Loan Need

1 1

3 3

5 4

Cash left = 1

UNSAFE because

A can finish: AVAIL = 2

but no one else can

No

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A l i th B k Al ith


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Needs

Customer Request Capital A B C D E Granted?

20 10 6 8 6 10

A 5 15 5 6 8 6 10 yes

B 3 12 5 3 8 6 10 yes

C 4 8 5 3 4 6 10 yes

D 3 5 5 3 4 3 10 yes

E 4 1 5 3 4 3 6 no

C 2 3 5 3 2 3 10 yesA 2 1 3 3 2 3 10 no

B 2 1 5 1 2 3 10 yes

D 1 0 5 1 2 2 10 no

B 1 0 5 0 2 3 10 yes6 5 - 2 3 10

E 4 2 5 - 2 3 6 yes

A 2 0 3 - 2 3 10 no

D 1 1 5 - 2 2 6 no

Applying the Bankers Algorithm

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General Case

Many resources of many types!

Banker has many currencies!

AVAIL and REQUEST are vectors.

one entry for each resource type

AVAILm and REQUESTm

So are ALLOCATEDi and NEEDi.

ALLOCATEDim

NEEDim

NEEDi AVAIL is true if for all m

NEEDim AVAILm10/1999


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Special Case:One resource of each type

A request will not be granted if a directed cycle would be created.

A

B

I II

A

B

I II

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Combined Approach

Internal resources PCBs, page tables, process headers,

can be ordered, prevention

CPU, real memory prevention via deny no preemption

Swap space

prevention, allocate all at the beginning

Non-shareable job resources dedicated disk drives, tapes

avoidance, or prevention, allocate all together

Shareable job resources disks; make shareable through drivers

Logical job resources

locks; detection (and recovery) 10/1999

Documents

Deadlock 6