Deadlock

Tanenbaum Ch 6

Silberschatz Ch 7

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The Deadlock Problem

• A Computing example: record scanned document

• System has 1 scanner and 1 CD Recorder

• 2 processes each need both devices.

P0 acquires scanner and waits for CD

P1 acquires CD and waits for scanner

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Preemptable and Nonpreemptable Resources

Sequence of events required to use a resource:

1. Request the resource.

2. Use the resource.

3. Release the resource.

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-1. Using a semaphore to protect resources. (a) One resource. (b) Two resources.

Resource Acquisition (1)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-2. (a) Deadlock-free code.

Resource Acquisition (2)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-2. (b) Code with a potential deadlock.

Resource Acquisition (3)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Introduction To Deadlocks

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.Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Deadlock Characterization:Necessary Conditions

• Mutual Exclusion:– At least 1 resource must be held in a non-sharable

mode.

• Hold and wait:– There must be at least 1 process that is holding one

resource and is waiting for another

• No preemption:– Resources cannot be preempted

• Circular wait:– There must be a cycle of dependency among a set of

processes and resources.

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Deadlock StrategiesStrategies for dealing with deadlocks:1. Just ignore the problem.

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

3. Dynamic avoidance by careful resource allocation.

4. Prevention, by structurally negating one of the four required conditions.

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Ignore the Problem

• May not occur very often

• May not have serious consequences if it does happen

• May be difficult / expensive to detect.

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Resource Allocation Graphs

Process

Resource

Request a Resource

Hold a Resource

Deadlock

R

A

A

A

B

AR

R

R S

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Resource Allocation GraphMultiple Resources with No Cycles

P1 P2 P3

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Graph with Deadlock

P1 P2 P3

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Graph with Deadlock

P1 P2 P3

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Graph with Cycle but no Deadlock

P1

P4

P3

P2

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Figure 6-6. The four data structures needed by the deadlock detection algorithm.

Deadlock Detection with Multiple Resources of Each Type

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Deadlock Detection with Multiple Resources of Each Type (2)

Deadlock detection algorithm:1. Look for an unmarked process, Pi , for which

the i-th row of R is less than or equal to A.

2. If such a process is found, add the i-th row of C to A, mark the process, and go back to step 1.

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

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-7. An example for the deadlock detection algorithm.

Deadlock Detection with Multiple Resources of Each Type (3)

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Deadlock AvoidanceDeadlock Avoidance

• The previous approach can be used to avoid a deadlock (rather than just detect one when it occurs).– Declare the maximum number of resources of each

type that will be required before the process starts.– Before any resource allocation is made, ensure that

the allocation will not leave the system in a state where a deadlock can occur.

– This will ensure that a process can always get access to its needed maximums.

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Safe State

• A system is in a safe state if there exists a sequence of resource assignments such that each process can be allocated its maximum request, and then release the resources, and then allow remaining processes to do the same...

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Safe State• A system is in a safe state if there exists a sequence of

resource assignments such that each process can be allocated its maximum request, and then release the resources, and then allow remaining processes to do the same...

Process Max Needs Current Needs Total

P0 10 5 12

P1 4 2 Avail

P2 9 2 3

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Safe State• A system is in a safe state if there exists a sequence of

resource assignments such that each process can be allocated its maximum request, and then release the resources, and then allow remaining processes to do the same...

Process Max Needs Current Needs Total

P0 10 5 12

P1 4 24 Avail

P2 9 2 31

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Safe State• A system is in a safe state if there exists a sequence of

resource assignments such that each process can be allocated its maximum request, and then release the resources, and then allow remaining processes to do the same...

Process Max Needs Current Needs Total

P0 10 5 12

P1 0 40 Avail

P2 9 2 15

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Safe State

• A system is in a safe state if there exists a sequence of resource assignments such that each process can be allocated its maximum request, and then release the resources, and then allow remaining processes to do the same...

Process Max Needs Current Needs Total

P0 10 510 12

P1 0 0 Avail

P2 9 2 50

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Banker's Algorithm

• Use vectors and matrices to identify resource requirements– Available[j] = k; //resource j has k members available– Max[i,j] = k; //Process i may need k members of

resource j– Allocation [i,j] = k; //Process i is currently allocated k

members of resource j– Need[i,j] = k; // Process i may need as many as k

more of resource j.

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Banker's ExampleRequest: P0 wants 0 2 0

Allocate Max Avail.

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

Safe order: ???

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

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

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 3 0 2 3 2 2 2 3 0

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 3 2 2 3 2 2 2 3 0

P2 3 0 2 9 0 2 2 1 0

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

Safe Order: P1,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 0 0 0 3 2 2 2 3 0

P2 3 0 2 9 0 2 2 1 0

P3 2 1 1 2 2 2 5 3 2

P4 0 0 2 4 3 3

Safe Order: P1,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2

P1 0 0 0 3 2 2 2 3 0

P2 3 0 2 9 0 2 2 1 0

P3 2 2 2 2 2 2 5 3 2

P4 0 0 2 4 3 3 5 2 1

Safe Order: P1, P3,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2 7 4 3

P1 0 0 0 3 2 2 2 3 0

P2 3 0 2 9 0 2 2 1 0

P3 0 0 0 2 2 2 5 3 2

P4 0 0 2 4 3 3 5 2 1

Safe Order: P1, P3,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2 7 4 3

P1 0 0 0 3 2 2 2 3 0 3 1 2

P2 3 0 2 9 0 2 2 1 0

P3 0 0 0 2 2 2 5 3 2

P4 4 3 3 4 3 3 5 2 1

Safe Order: P1, P3, P4,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 1 0 7 5 3 3 3 2 7 4 3

P1 0 0 0 3 2 2 2 3 0 3 1 2

P2 3 0 2 9 0 2 2 1 0 7 4 5

P3 0 0 0 2 2 2 5 3 2

P4 0 0 0 4 3 3 5 2 1

Safe Order: P1, P3, P4,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 7 5 3 7 5 3 3 3 2 7 4 3

P1 0 0 0 3 2 2 2 3 0 3 1 2

P2 3 0 2 9 0 2 2 1 0 7 4 5

P3 0 0 0 2 2 2 5 3 2 0 0 2

P4 0 0 0 4 3 3 5 2 1

Safe Order: P1, P3, P4, P0,

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Banker's Example (change 1)

P1 requests (1 0 2)

Allocate Max Avail.

A B C A B C A B C

P0 0 0 0 7 5 3 3 3 2 7 4 3

P1 0 0 0 3 2 2 2 3 0 3 1 2

P2 3 0 2 9 0 2 2 1 0 7 4 5

P3 0 0 0 2 2 2 5 3 2 0 0 2

P4 0 0 0 4 3 3 5 2 1 7 5 5

Safe Order: P1, P3, P4, P0, P2 !!! OK

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Banker's Example (change 2)

P4 requests (3 3 0)

Allocate Max Avail.

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

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Banker's Example (change 2)

P4 requests (3 3 0)

Allocate Max Avail.

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 0 0 2

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 3 3 2 4 3 3

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Banker's Example (change 2)

P4 requests (3 3 0)

Allocate Max Avail.

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 0 0 2

P2 3 0 2 9 0 2

P3 2 1 1 2 2 2

P4 0 0 2 4 3 3

No job can be completed. No Safe Order exists!

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Recovery from Deadlock

• Recovery through preemption

• Recovery through rollback

• Recovery through killing processes

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Deadlock Prevention

• Attacking the mutual exclusion condition

• Attacking the hold and wait condition

• Attacking the no preemption condition

• Attacking the circular wait condition

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-13. (a) Numerically ordered resources. (b) A resource graph.

Attacking the Circular Wait Condition

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Figure 6-14. Summary of approaches to deadlock prevention.

Approaches to Deadlock Prevention

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Other Issues

• Two-phase locking

• Livelock

• Starvation

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Locking Protocols

• Shared Locks (read but not write)• Exclusive Locks (read or write)

• A transaction may require a number of locks that must be applied consistently (same sequence..)

• 2 Phase locking protocol– Growing Phase (obtain but not release locks)– Shrinking Phase (release but not obtain locks)

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Figure 6-16. Busy waiting that can lead to livelock.

Livelock

Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

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Starvation

• Occurs when there is a priority based selection process.

• If there are many high priority tasks (in a continuous stream) and only one (or a few) low priority tasks, the low priority tasks will never be selected

• Starvation avoidance– First-come, First-served– Priority enhancement

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Summary• Deadlock Characteristics

– Mutual Exclusion:– Hold and wait:– No preemption:– Circular wait:

• Deadlock Strategies1. Just ignore the problem. 2. Detection and recovery. Let deadlocks occur, detect them,

take action.3. Dynamic avoidance by careful resource allocation.4. Prevention, by structurally negating one of the four required

conditions.• Deadlock Tools and Techniques• Other Issues

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Questions

• In the context of Operating System processes, what is a deadlock? What conditions are required for a deadlock to exist?

• How does a resource allocation graph help identify opportunities for deadlock? Please show an example of a resource allocation graph that includes a deadlock.

• Discuss how the Banker's algorithm ensures that each process that tries to get access to a critical section will eventually be guaranteed of getting in.

• The Banker’s algorithm only requires that there be at least 1 safe order to make a transaction safe, however there may be many different orders possible. Using the example shown on slide 34, how many safe orders exist given the request by P1?

• How might we handle deadlocks? Give at least 3 of the methods discussed in the book and provide examples of each.