Operating Systems

04/20/23 CST 352 - Operating Systems 1

Operating Systems

CST 352

Deadlock


Topics

Definitions

Deadlock Conditions

Ostrich Algorithm

Deadlock Detection

Deadlock Recovery

Deadlock Avoidance

Deadlock Prevention


Definitions

Resources – Parts of a system that are required for a process to run (e.g. memory, disk, CPU, etc.)

Preemptable – A resource that can be taken from a process without adverse affects (e.g. seized memory)Non Preemtable– A resource that cannot be taken away (e.g. a device driver connected to printer)


Definitions

Deadlock – In a set of processes, each process is waiting for a resource that one of the other process has seized.

Remember the dining philosophers.


Definitions

A Traffic Deadlock

All Stop…..

Ready…..

Go!


Definitions

A Traffic DeadlockOops….

Deadlock

How is this deadlock avoided under normal circumstances?


Definitions

Question:

1. Which of the two types of resource can lead to a deadlock condition?

2. Why?


Definition

Question:

Can there be deadlock in a “first-come, first-serve” scheduling scheme?


Required Conditions

Four conditions necessary for Deadlock:1. Mutual Exclusion – Only one process can

access a resource at any given time.

2. Hold and Wait – A process may hold a resource while waiting for others to become available.

3. No preemption – No resource may be forcibly taken from a holding process.

4. Circular wait – There must be at least two processes, each waiting for a resource held by the other.


Deadlock Concepts

Modeling Deadlock (grid method)

X – axis: progress of process A.

Y – axis: progress of process B.

This gives us a good feel for deadlock overlap, however, it becomes unmanageable for more than two processes.

04/20/23 CST 352 - Operating Systems 11Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1

Rel 1Get 1 Get 2 Rel 2

Proc A andProc B wantResource 2


DeadlockPending

Deadlock ConceptsModeling

Deadlock (Resource Trajectories)

Example: Proc A and Proc B competing for Resource 1 and Resource 2.


Deadlock Concepts

Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1




DeadlockPending

5. B acquires 1

6. B releases 2

7. B releases 1

8. A can acquire both 1 and 2

4. B acquires 2

1. A Executes

2. B Executes

3. A Executes


Deadlock Concepts

Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1




DeadlockPending

5. B acquires 1

6. A blocks on 1

7. B releases 2

9. A can acquire both 1 and 2

4. B acquires 2

1. A Executes

2. B Executes

3. A Executes

8. B releases 1


Deadlock Concepts

Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1




DeadlockPending

5. A acquires 1

6. B blocks on 1

7. A blocks on 2

4. B acquires 2

1. A Executes

2. B Executes

3. A Executes

8. Deadlock

X


Deadlock Concepts

Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1

Get 2Get 1 Rel 1 Rel 2

Proc A andProc Bwant

Resource 2

Proc A andProc Bwant

Resource 1

To reduce resource contention, construct Process A such that it does not need both resource 1 and resource 2 at the same time.

This increases the number of valid paths through the “gray” areas.


Deadlock Concepts

Modeling Deadlock (grid method)

The resource trajectory model has disadvantages

Unmanageable for complex systems

Dependent on resource acquisition sequencing

This model does suggest solutions to deadlock (e.g. avoid getting into the “Deadlock Pending” region.


Deadlock ConceptsModeling Deadlock (graph method)

A circle represents a processA square represents a resourceA directed arch from a circle to a square represents a process requesting a resource.

P1 requesting R1

A directed arch from square to circle represents a process holding a resource

P1 holding R1

A Deadlock is condition is represented by a “cycle” in the graph.

P1 R1

P1 R1


Deadlock Concepts

Modeling Deadlock (graph method)

Example:Processes P1, P2, and P3

Resources R1, R2, and R3

P1

R1

P3P2

R2 R3


Deadlock Concepts

Modeling Deadlock (graph method)

P1

R1

P3P2

R2 R3

1. P1 requests and holds R1



4. P1 requests R2

5. P2 requests R3

6. P3 requests R1

The cyclic condition of the graph indicates this sequence of event will cause deadlock.


Ostrich Algorithm

Stick your head in the sand and pretend there is no problem at all.Mathematically speaking – this is unacceptable.From an engineering point of view, this may be ok.


Ostrich Algorithm

Considerations:

What is the half-life of your OS.

What resource contentions could cause a deadlock.

How often statistically does your OS experience a visible deadlock.


Ostrich Algorithm

The Ostrich Algorithm is the easiest solution to the deadlock problem.In standard “user” based systems (e.g. Windows, Unix Workstations, etc.), this is perfectly acceptable.Would you consider the “Ostrich Algorithm” an acceptable solution for an enterprise management system?


Deadlock Detection

Graph based approachFor each resource access

Create a node representing the process accessing the resource.Create a node representing the resource.Make the appropriate directed association.


Deadlock Detection

Graph based approach (example)1. PA requests R1 and gets it.2. PB requests R2 and gets it.3. PC requests R2.

4. PD requests R3 and gets it.5. PD requests R1.

PA R1

PB R2

PC R3 PD

6. PA requests R3.

Every time a resource request is made:

1. Add the appropriate node to the graph

2. Check the graph for cycles.


Deadlock Detection

Graph based approachTo implement the graph based approach, the OS needs to:

Build a graph on the fly based on resource requests.Provide some form of cycle detection to detect deadlock conditions in the constructed graph.


Deadlock Detection

Q:

1. List the four conditions for deadlock.


Deadlock Detection

Matrix ApproachLet “n” = number of processes == (P1, …, Pn).Let “m” = number of resource classes.

E1 – resource of type 1

E2 – resource of type 2…Em – resource of type m


Deadlock Detection

Matrix ApproachAt any point in time, the allocation of resources can be represented by a two vectors.

(E1, E2, …, Em) – Existing resource vector.

(A1, A2, …, Am) – Available resource vector.


Deadlock Detection

Matrix ApproachRepresent allocation with a matrix

Columns represent resourcesRows represent processes

Represent requests with a matrixColumns represent resourcesRows represent processes


Deadlock Detection

Matrix Approach

Current Allocation and Requestc11 c12 … c1m

c22 c22 … c2m

… … …

cn1 cn2 … cnm

pro

cess

Current Allocation

r11 r12 … r1m

r22 r22 … r2m

… … …

rn1 rn2 … rnm

pro

cess

Current Request


Deadlock Detection

Matrix Approach

At any point in time, the resource state must conform to the equation:

For any j (j -> resource “column”):

i = 1

n

cij+Aj = Ej

e.g. allocated instances of resource j + available instances of resource j = number of existing instances of resource j.


Deadlock Detection

Matrix ApproachProcess allocation is now done by comparing vectors.

Vector A <= B iffFor all i < m, Ai <= Bi

Initialize the processes to “r” for “run”.As the algorithm proceeds, set the process flags to either “r” for processes capable of running, or “b” for processes blocked and waiting for a resource.


Deadlock Detection

Matrix ApproachEvery time there is a resource allocation:

Look for a process (Pi) for which the request row (Ri) <= available resources (Ai).

Looking for resource demands that can be met.

If such a process is found, add the ith row of the c matrix to the A vector.

Loop over next request.

ElseTerminate the detection algorithm and set Pi to “b”

All “b” processes are blocked.


Deadlock Detection

Matrix ApproachIn practical application, keep the matrices as a running total.

Each time there is a resource allocation, update the matrices.

Each time there is a resource de-allocation, update the matrices.

Deadlock occurs when two (or more) processes have cross referenced resources.


Deadlock Detection

Matrix Approach (Example)

Resource Vector Available Vector

E = [3 2 2] A = [3 2 2]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

0 0 00 0 00 0 0

2 0 00 0 00 0 0

C = R =P1P2P3

1) P1 requests 2 Printers

Current Allocation Requestrrr


Deadlock Detection



E = [3 2 2] A = [1 2 2]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 00 0 0

0 0 00 0 00 0 0

C = R =P1P2P3



Deadlock Detection



E = [3 2 2] A = [1 2 2]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 00 0 0

0 0 01 0 20 0 0

C = R =P1P2P3

2) P2 requests 1 Printer, 2 DVDs



Deadlock Detection



E = [3 2 2] A = [0 2 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 01 0 20 0 0

0 0 00 0 00 0 0

C = R =P1P2P3



Deadlock Detection



E = [3 2 2] A = [0 2 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 01 0 20 0 0

0 0 00 0 01 1 0

C = R =P1P2P3

3) P3 requests 1 Printer, 1 CD Rom



Deadlock Detection



E = [3 2 2] A = [0 2 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 01 0 20 0 0

0 0 00 0 01 1 0

C = R =P1P2P3

4) P3 blocked

Current Allocation Requestrrb


Deadlock Detection



E = [3 2 2] A = [0 2 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 01 0 20 0 0

0 0 00 0 01 1 0

C = R =P1P2P3

5) P2 frees up printer



Deadlock Detection



E = [3 2 2] A = [1 2 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 20 0 0

0 0 00 0 01 1 0

C = R =P1P2P3



Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 00 0 00 0 0

C = R =P1P2P3


6) P3 can now run


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 10 0 00 0 0

C = R =P1P2P3


7) P1 request DVD


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 10 0 00 0 0

C = R =P1P2P3

Current Allocation Requestbrr

8) P1 blocked


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 11 0 00 0 0

C = R =P1P2P3

Current Allocation Requestbrr

9) P2 request printer


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 11 0 00 0 0

C = R =P1P2P3

Current Allocation Requestbbr

10) P2 blocked


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 11 0 00 0 0

C = R =P1P2P3


• This is still not deadlock since P3 holds a resource that could allow P2 to commence execution.

• If P1 held all 3 printers, we would have true deadlock.


Deadlock Detection



E = [3 2 2] A = [0 1 0]

Pri

nte

r

CD

Rom

DV

D

Pri

nte

r

CD

Rom

DV

D

2 0 00 0 21 1 0

0 0 11 0 00 0 0

C = R =P1P2P3


• What are the general conditions for true deadlock?


Deadlock Detection

Hypothesis (extra credit – 50 pts)Prove or disproveDeadlock can be detected by rotating the

rows and columns of the C and R matrix (in conjunction) so that if C and R have any inverse sub-matrices, the involved processes are deadlocked.

E and A must be involved.


Deadlock Recovery

Given the above algorithm, the OS can detect which process is waiting for what resource.

Now the OS needs to decide what to do when a deadlocked process is detected.


Deadlock Recovery

Recovery through preemptionTake a resource away from a process and give it to another.

Only works for preemptable processes.


Deadlock Recovery

Recovery through rollbackPCB (TCB) and resource state are periodically saved at a “checkpoint”

When a deadlock is detected, back the deadlocked process up to the previous stable state.

Discard the resource manipulation that occurred after the checkpoint.

Start the process after it is determined it can run again.


Deadlock Recovery

Recovery through process killing

Kill off the offending processes.The processes that have allocated contending resources are removed from the system.

Kill off the lower priority process.

Kill off non-OS (user) processes.

Return their resources to the system.


Deadlock Recovery

Q:1. How can a system be constructed

so it will never have deadlock?2. Give an example.3. T or F – The graph model for

deadlock only works well for 2 processes.


Deadlock Avoidance

Process Trajectories – revisited

Process A

Pro

cess

B

Get 2

Rel 1

Rel 2

Get 1




DeadlockPending

Shaded regions with slanted lines represent instances where both processes gain access to a resource at the same time.

If the resources are mutually exclusive, it is impossible for these areas to be entered by a “run time trajectory”.


Deadlock Avoidance

Bankers AlgorithmThe banker at Klamath First Federal is granting a group of customers credit.The banker has limited cash to loan.Each customer has a credit limit.The banker assumes no customer will request their entire credit limit at a single time.


Deadlock Avoidance

Bankers AlgorithmThe banker grants loans to the customers based customer credit limit and available cash reserve.The bankers job is to avoid the situation where cash reserves drop so low that the FDIC shuts the bank down.Loans that may cause inspection by the FDIC are called “unsafe” loans.


Deadlock Avoidance

Bankers Algorithm (Example)4 customers – Joe, Jay, Jim, and JillInitial reserve of 10 dollars (it is a very small bank).Each customer has a credit limit.

The goal here is to ensure that two customers cannot be simultaneously blocked waiting on a resource.


Deadlock Avoidance

Bankers Algorithm (Example)

Name LimitHas

Joe

Jay

Jim

Jill

0

0

0

0

6

7

4

5

Reserve: 10

1. Joe requests 1


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

0

6

7

4

5

Reserve: 9

1. Joe requests 1 – granted


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

0

6

7

4

5

Reserve: 9

1. Joe requests 1 – granted2. Jay requests 1


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

1

6

7

4

5

Reserve: 8

1. Joe requests 1 – granted2. Jay requests 1 – granted


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

1

6

7

4

5

Reserve: 8

1. Joe requests 1 – granted2. Jay requests 1 – granted3. Jim requests 2


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

1

6

7

4

5

Reserve: 6

1. Joe requests 1 – granted2. Jay requests 1 – granted3. Jim requests 2 – granted


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

1

6

7

4

5

Reserve: 6

1. Joe requests 1 – granted2. Jay requests 1 – granted3. Jim requests 2 – granted4. Jill requests 4


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

4

2

1

6

7

4

5

Reserve: 2

1. Joe requests 1 – granted2. Jay requests 1 – granted3. Jim requests 2 – granted4. Jill requests 4 – granted


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

4

2

1

6

7

4

5

Reserve: 2

At any point in time, in this scenario, the states are safe because, with at least two units left, the banker can delay granting any requests until Jim pays back his loan. Jim can ask for the additional 2 dollars.


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

4

0

1

6

7

4

5

Reserve: 4

When Jim pays back his loan, the banker can lend to Jay or Jill.


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

2

6

7

4

5

Reserve: 7

1. Joe requests 1 – granted2. Jay requests 2 – granted

Unsafe State Example


Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

0

2

6

7

4

5

Reserve: 7

1. Joe requests 1 – granted2. Jay requests 2 – granted3. Jim requests 2



Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

2

6

7

4

5

Reserve: 5

1. Joe requests 1 – granted2. Jay requests 2 – granted3. Jim requests 2 – granted



Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

2

6

7

4

5

Reserve: 5




Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

2

6

7

4

5

Reserve: 5


With a reserve of 1, potentially, any other request could be denied. If the banker gives the

loan to Jill, the reserve will be down to 1.



Deadlock Avoidance


Name LimitHas

Joe

Jay

Jim

Jill

1

0

2

2

6

7

4

5

Reserve: 5


Granting Jill the loan would force the system into an unsafe state.

If all customers demanded their maximum loan, none of them could be granted, forcing the FDIC to come down on the bank (deadlock).


Deadlock Avoidance

Bankers Algorithm (multiple resource)The bankers algorithm for multiple resource is just a modification of the matrix approach for deadlock detection.The modification made relates to what the scheduler does during the application of the algorithm.Rather than waiting for deadlock to occur, the scheduler takes proactive action during resource allocation and de-allocation.


Deadlock Avoidance

Bankers Algorithm (multiple resource)The problem with the bankers algorithm for deadlock avoidance is to operate correctly, the resources must be pre-determined and stable.

There are no systems where resources are all pre-determined and infinitely stable.


Deadlock Prevention

Attack mutual exclusion condition

Do not allow any resources to be mutually exclusive.

Printer spooling was introduced to do this.


Deadlock Prevention

Attack the hold and wait conditionDo not allow any process to seize a resource and hold it.

Require all processes to request their resources up-front.

This solution introduces inefficiencies.


Deadlock Prevention

Attack the No-Preempt condition

It is close to impossible to get rid of all non-preemptable resources in a system.


Deadlock Prevention

Attack the Circular wait condition

Only allow a process one resource at a time.Require process to adhere to a resource numbering scheme sequential allocation

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