june 13 deadlock

7/30/2019 June 13 Deadlock

1/14

Chapter 81

Deadlock is: A set of blocked processes each holding a

resource and waiting to acquire a resource

held by another process in the set.

Example (hardware resources):

System has 2 tape drives.

P1 and P2 each hold one tape drive and each

needs another one.

Example (software resource:):

semaphoresA

andB

, initialized to 1P0 P1

wait (A); wait(B)

wait (B); wait(A)

signal(B); signal(A)

signal(A); signal(B)


2/14

Chapter 82

The (4) Conditions for Deadlock

There are three policy conditions that must hold fora deadlock to be possible(the necessary conditions):

1. Mutual Exclusion

only one process at a time may use a resource

2. Hold-and-Wait

a process may hold allocated resources while awaiting

assignment of others

3. No Preemption

a resource can be released only voluntarily by the

process holding it, after that process has completed its

task

..and a fourth condition which must actually arise

to make a deadlock happen


3/14

Chapter 83

The Conditions for Deadlock

In the presence of these necessary conditions,onemore condition must arise for deadlock to actually

occur:

4.Circular Wait

a closed chain of processes exists, such that each

process holds at least one resource needed by the next

process in the chain


4/14

Chapter 84

The Conditions for Deadlock

Deadlock occurs if and only ifthe circular

wait condition is unresolvable

The circular wait condition is unresolvable

if the first 3 policy conditions hold

So the 4 conditions taken togetherconstitute necessary and sufficient

conditions for deadlock


5/14

Chapter 85

Resource Allocation Graph:A Cycle But No Deadlock

Multiple

instances

of resources R1and R2:

P2 and P4 can

complete,

freeing

up resources for

P1 and P3


6/14

Chapter 86

Approaches to Handling Deadlocks

Deadlock Prevention

disallow 1 of the 3 necessary conditions ofdeadlock occurrence, or prevent the circularwait condition from happening

Deadlock Avoidance

do not grant a resource request if thisallocation might lead to deadlock

Deadlock Detection and Recovery grant resource requests when possible, but

periodically check for the presence of deadlockand then take action to recoverfrom it


7/14Chapter 87

Deadlock Avoidance

We accept the 3 policy conditions but makejudicious choices to assure that the deadlock

point is never reached

Allows more concurrency than prevention

Two approaches: do not start a process if its total demand might lead to

deadlock: (Process Initiation Denial), or

do not grant an incremental resource request if this

allocation couldlead to deadlock: (Resource Allocation

Denial)

In both cases: maximum requirements of each

resource must be stated in advance


8/14Chapter 88

Resource Allocation Denial

A Better Approach:

Grant incremental resource requests if we can prove that

this leaves the system in a state in which deadlock

cannot occur.

Based on the concept of a safe state

Bankers Algorithm:

Tentatively grant each resource request

Analyze resulting system state to see if it is safe.

Ifsafe, grant the request

ifunsafe refuse the request (undo the tentative grant)

block the requesting process until it is safe to grant it.


9/14

Data Structures for the Bankers Algorithm

Let n = number of processes,

m = number of resource types

Available: Vectorof length m. IfAvailable [j] = k, there are k

instances of resource type Rj currently available

Max:nxmmatrix. IfMax[i ,j] = k, then process Piwill request atmost kinstances of resource type Rj.

Al loc:nxmmatrix. IfAlloc[i ,j] = kthen Piis currently allocated(i.e. holding) k instances ofRj.

Need:nxmmatrix. IfNeed[i ,j] = k, then Pimay need kmoreinstances ofRjto complete its task.

Need[i,j]= Max[i ,j]Al loc[i ,j].


10/14Chapter 810

Safety Algorithm

1. Let Workand Finishbe vectors of length mand n,respectively. Initialize:

Work:= Available

Finish[i] == falsefor i= 1,2, , n.

2. Find an isuch that both:

Finish[i] == false

NeediWorkIf no such iexists, go to step 4.

3. Work := Work+ Allocationi

(Resources freed when process completes!)Finish[i] := true

go to step 2.

4. IfFinish[i] = true for all i, then the system is in a

safe state.


11/14Chapter 811

Resource-Request Algorithm for Process Pi

Requesti = request vector for Pi .

Requesti [j] = kmeans process Pi wants k instances of resource

type Rj.

1. IfRequesti Needi go to step 2. Otherwise, error ( processexceeded its maximum claim).

2. IfRequesti

Available, go to step 3. Otherwise Pi mustwait, (resources not available).

3. Allocate requested resources to Pi as follows:

Available := Available - Requesti

Alloci := Alloci + Requesti

Needi

:= Needi

RequestiIfsafe the resources are allocated to Pi.

Ifunsafe restore the old resource-allocation state andblock Pi


12/14Chapter 812

Example of Bankers Algorithm

5 processesP0 throughP43 resource typesA (10 units),B (5 units), and C(7

units).

Snapshot at time T0:

Al location 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 2P4 0 0 2 4 3 3


13/14

Chapter 813

Example (cont)

Need = Max

AllocationNeed

A B C

P0 7 4 3

P1 1 2 2

P2 6 0 0

P3 0 1 1

P4 4 3 1

The system is in a safe state since the sequence satisfies safety criteria.


14/14

Chapter 814

Now P1 requests (1,0,2)

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 2 6 0 0

P3 2 1 1 0 1 1

P4 0 0 2 4 3 1

june 13 deadlock

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