1. A. Berrached::cms::uhd
Deadlock
Chapter 10
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2. A. Berrached::cms::uhd
Deadlock
System Model
Deadlock Characterization
Methods for Handling Deadlocks
Deadlock Prevention
Deadlock Avoidance
Deadlock Detection & Recovery
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3. A. Berrached::cms::uhd
The Deadlock Problem
A state where a set of processes are blocked each holding a resource and waiting to acquire a resource held by another process in the set.
Example: Semaphores A and B initialized to 1
P0: P1:
wait(A); wait(B);
wait(B); wait(A);
…. …..
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4. A. Berrached::cms::uhd
Process 1 Process 2 Process 3
…. …. …...
Request (resource1); Request (resource2); Request (resource3); /*holding res 1 */ /*holding res 2 */ /*holding res 3 */
….. ….. …..
Request(resource2) Request(resource3); Request(resource1);
…. ….. …..
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5. A. Berrached::cms::uhd
System Model
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6. Deadlock Characterization
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7. Resource Allocation Graph
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8. Resource Allocation Graph (cont.)
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9. Example of a graph with no cycles
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10. Example of a graph with a cycle
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11. A. Berrached::cms::uhd
Basic Facts
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12. Methods for handling deadlocks
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13. A. Berrached::cms::uhd
Deadlock Prevention
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14. Deadlock Prevention (cont.)
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15. A. Berrached::cms::uhd
Deadlock Avoidance
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16. A. Berrached::cms::uhd
Safe State
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17. A. Berrached::cms::uhd
Basic Facts
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18. A. Berrached::cms::uhd
Banker's Algorithm
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19. Data Structures for the Banker's Algorithm
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20. A. Berrached::cms::uhd
Safety Algorithm
Else if no i can be found where Finish[i]=flase and
Needi <= Work , then the system is in an unsafe state
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21. Resource Request Algorithm for Process Pi
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22. Example of Banker's Algorithm
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23. A. Berrached::cms::uhd
Example (cont.)
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24. Example (cont.): P1 requests (1,0,2)
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25. A. Berrached::cms::uhd
Deadlock Detection
Allow system to enter deadlock state
Detection Algorithm
Recovery scheme
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26. Single Instance of Each Resource Type
Maintain a resource allocation graph
Periodically invoke algorithm that searches for cycles in the graph
If there is a cycle ==> set of processes in cycle are in deadlock
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27. A. Berrached::cms::uhd
Deadlock Detection
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28. RAG Reduction Algorithm
Given the resource allocation graph of the system at the current state. Determine if the system is in deadlock.
1. if a process is not blocked (i.e. if all resources the process is requesting to finish its task are available), then reduce graph by that process: remove the process node from the graph, return all its allocated resources, and remove all arrows from and to that process.
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29. Deadlock Detection--RAG Reduction
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30. Deadlock Detection--RAG Reduction
2. Repeat step 1 until all process node are removed or the graph can not be reduced any further.
If a graph can be reduce by all its processes, then there is no deadlock
If a graph can NOT be completely reduced by all its processes then the "irreducible" processes constitute the set of deadlocked processes.
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31. A. Berrached::cms::uhd
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32. Deadlock Detection-- Usage
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33. Recovery From Deadlock
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34. Recovery From Deadlock
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