1. Tanenbaum Ch 6 Silberschatz Ch 7
Deadlock
Tanenbaum Ch 6
Silberschatz Ch 7

2. The Deadlock Problem A Computing example: record scanned document
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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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3. Preemptable and Nonpreemptable Resources
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Preemptable and Nonpreemptable Resources
Sequence of events required to use a resource:
Request the resource.
Use the resource.
Release the resource.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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4. Resource Acquisition (1)
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Resource Acquisition (1)
Figure 6-1. Using a semaphore to protect resources. (a) One resource. (b) Two resources.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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5. Resource Acquisition (2)
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Figure 6-2. (a) Deadlock-free code.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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6. Resource Acquisition (3)
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Resource Acquisition (3)
Figure 6-2. (b) Code with a potential deadlock.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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7. Introduction To Deadlocks
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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.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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8. Deadlock Characterization: Necessary Conditions
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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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9. Deadlock Strategies Strategies for dealing with deadlocks:
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Deadlock Strategies
Strategies for dealing with deadlocks:
Just ignore the problem.
Detection and recovery. Let deadlocks occur, detect them, take action.
Dynamic avoidance by careful resource allocation.
Prevention, by structurally negating one of the four required conditions.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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10. 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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11. Resource Allocation Graphs
Process
Resource
Request a Resource
Hold a Resource
Deadlock A R R A A R A R S B
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12. Resource Allocation Graph Multiple Resources with No Cycles
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Resource Allocation Graph Multiple Resources with No Cycles P2 P3 P1
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13. 4/16/2017
Graph with Deadlock P2 P3 P1
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14. 4/16/2017
Graph with Deadlock P2 P3 P1
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15. Graph with Cycle but no Deadlock
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Graph with Cycle but no Deadlock P2 P1 P3 P4
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16. Deadlock Detection with Multiple Resources of Each Type
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Deadlock Detection with Multiple Resources of Each Type
Figure 6-6. The four data structures needed by the deadlock detection algorithm.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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17. Deadlock Detection with Multiple Resources of Each Type (2)
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Deadlock Detection with Multiple Resources of Each Type (2)
Deadlock detection algorithm:
Look for an unmarked process, Pi , for which the i-th row of R is less than or equal to A.
If such a process is found, add the i-th row of C to A, mark the process, and go back to step 1.
If no such process exists, the algorithm terminates.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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18. Deadlock Detection with Multiple Resources of Each Type (3)
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Figure 6-7. An example for the deadlock detection algorithm.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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19. Deadlock 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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20. 4/16/2017
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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21. Safe State Process Max Needs Current Needs Total P0 10 5 12
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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 P
P Avail P
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22. Safe State Process Max Needs Current Needs Total P0 10 5 12
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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 P
P 4 Avail
P 1
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23. Safe State Process Max Needs Current Needs Total P0 10 5 12
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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 P
P 0 Avail
P 5
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24. Safe State Process Max Needs Current Needs Total P0 10 510 12
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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
P 
P Avail
P 0
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25. 4/16/2017
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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26. Banker's Example Request: P0 wants 0 2 0
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Banker's Example Request: P0 wants
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe order: ???
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27. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
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28. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
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29. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1,
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30. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1,
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31. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3,
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32. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3,
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33. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3, P4,
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34. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3, P4,
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35. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3, P4, P0,
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36. Banker's Example (change 1)
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Banker's Example (change 1)
P1 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
Safe Order: P1, P3, P4, P0, P2 !!! OK
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37. Banker's Example (change 2)
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Banker's Example (change 2)
P4 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
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38. Banker's Example (change 2)
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Banker's Example (change 2)
P4 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
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39. Banker's Example (change 2)
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Banker's Example (change 2)
P4 requests ( )
Allocate Max Avail.
A B C A B C A B C P P P P P
No job can be completed. No Safe Order exists!
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40. Recovery from Deadlock
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Recovery from Deadlock
Recovery through preemption
Recovery through rollback
Recovery through killing processes
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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41. Deadlock Prevention Attacking the mutual exclusion condition
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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
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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42. Attacking the Circular Wait Condition
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Attacking the Circular Wait Condition
Figure (a) Numerically ordered resources. (b) A resource graph.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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43. Approaches to Deadlock Prevention
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Approaches to Deadlock Prevention
Figure Summary of approaches to deadlock prevention.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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44. Other Issues Two-phase locking Livelock Starvation cs431-cotter
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Other Issues
Two-phase locking
Livelock
Starvation
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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45. Locking Protocols Shared Locks (read but not write)
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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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46. Livelock Figure 6-16. Busy waiting that can lead to livelock.
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Livelock
Figure Busy waiting that can lead to livelock.
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Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved
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47. 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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48. Summary Deadlock Characteristics Deadlock Strategies
Mutual Exclusion:
Hold and wait:
No preemption:
Circular wait:
Deadlock Strategies
Just ignore the problem.
Detection and recovery. Let deadlocks occur, detect them, take action.
Dynamic avoidance by careful resource allocation.
Prevention, by structurally negating one of the four required conditions.
Deadlock Tools and Techniques
Other Issues
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49. 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.
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