1. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks 主講人：虞台文

3. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Deadlocks with Reusable and Consumable Resources

4. Deadlocks

8. Example (File Sharing) p1: ... open(f1,w); open(f2,w); ... p2: ... open(f2,w); open(f1,w); ... Deadlock when executed concurrently

9. Example (Message Passing) p1: ... if( C )send(p2,m); while(1){ recv(p3,m); ... send(p2,m); } Deadlock when C is not true p2: ... while(1){ recv(p1,m); ... send(p3,m); } p3: ... while(1){ recv(p2,m); ... send(p1,m); } Assume that send / recv are blocking operations.

12. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Approaches to the Deadlock Problem

14. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks System Model

16. State Transitions by p 1 Process S 0 S 1 S 2 S 3 The state transition diagram described below is not complete . The actual number of states depends on possible operations of processes. R p 1 p 2 R p 1 p 2 R p 1 p 2 R p 1 p 2 Request R (by p 1 ) Acquire R (by p 1 ) Release R (by p 1 )

19. Example p 1 and p 2 both need R 1 and R 2 . R 1 R 2 p 1 p 2 p1: Request( R1 ); Request( R2 ); Release( R2 ); Release( R1 ); p2: Request( R2 ); Request( R1 ); Release( R1 ); Release( R2 );

20. Example p1: Request( R1 ); Request( R2 ); Release( R2 ); Release( R1 ); p2: Request( R2 ); Request( R1 ); Release( R1 ); Release( R2 );

21. Example p1: Request( R1 ); Request( R2 ); Release( R2 ); Release( R1 ); p2: Request( R2 ); Request( R1 ); Release( R1 ); Release( R2 );

22. Example p1: Request( R1 ); Request( R2 ); Release( R2 ); Release( R1 ); p2: Request( R2 ); Request( R1 ); Release( R1 ); Release( R2 );

23. Example p1: Request( R1 ); Request( R2 ); Release( R2 ); Release( R1 ); p2: Request( R2 ); Request( R1 ); Release( R1 ); Release( R2 );

24. Example R 1 R 2 p 1 p 2

25. Example p 1 is blocked. p 2 is blocked.

26. Example A deadlock state. No state is safe since the deadlock state is always reachable .

27. Necessary Condition for Deadlock A cycle in the resource graph is a necessary condition for deadlock.

28. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Deadlock Detection

52. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Recovery from Deadlock

54. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Dynamic Deadlock Avoidance

60. The Banker’s Algorithm p 1 p 2 p 3 R 1 R 2

61. The Banker’s Algorithm Which of the requests can safely be granted ? Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ? R 1 R 2 p 1 p 2 p 3

62. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ? Temporarily grant Can such an RAG be completely reducible? R 1 R 2 p 1 p 2 p 3

63. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ? p 1 p 2 p 3 Temporarily grant Can such an RAG be completely reducible?  R 1 R 2

64. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ? granted Can such an RAG be completely reducible?  R 1 R 2 p 1 p 2 p 3

65. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?  R 1 R 2 p 1 p 2 p 3

66. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ? Can such an RAG be completely reducible? Temporarily grant   R 1 R 2 p 1 p 2 p 3

67. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?   not granted R 1 R 2 p 1 p 2 p 3

68. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?   R 1 R 2 p 1 p 2 p 3

69. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?   Can such an RAG be completely reducible? Temporarily grant R 1 R 2 p 1 p 2 p 3

70. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?   p 1 p 2 p 3 Can such an RAG be completely reducible? Temporarily grant  R 1 R 2

71. The Banker’s Algorithm Can p 1  R 1 be safely be granted ? Can p 2  R 1 be safely be granted ? Can p 3  R 1 be safely be granted ?    granted p 1 p 2 p 3 R 1 R 2

73. Operating Systems Principles Process Management and Coordination Lecture 6: Deadlocks Deadlock Prevention

79. Comparisons