Chapter 3: Deadlocks

1. Chapter 3: Deadlocks Instructor: Hengming Zou, Ph.D. In Pursuit of Absolute Simplicity求于至简，归于永恒

2. Content • Resource • Deadlocks • Deadlock modeling • Deadlock detection and recovery • Deadlock avoidance and prevention

3. Resources • Something needed by a thread • A thread waits for resources • Examples of computer resources • Printers, tape drives, tables, • locks, disk space, memory, CPU

4. Resources • Processes need access to resources • in reasonable order • Preemptable resources • can be taken away from a process with no ill effects • Nonpreemptable resources • will cause the process to fail if taken away

5. Resources • Suppose a process holds resource A • and requests resource B • At same time another process holds B • and requests A • both are blocked and remain so • Deadlock!

6. Resources • Sequence of events required to use a resource • request the resource • use the resource • release the resource

7. Resources • Must wait if request is denied • requesting process may be blocked • may fail with error code

8. Deadlocks • A circular waiting for resources • leading to threads involved not being able to make progress • Deadlocks occur when… • processes are granted exclusive access to resources

9. Deadlocks • Formal definition: • 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 • Usually the event is release of a held resource

10. Deadlocks • In a deadlock, none of the processes can … • run • release resources • be awakened

11. Deadlocks • General structure of thread code • phase 1. while (not done) { • acquire some resources • work • } • phase 2. release all resources • Assume phase 1 has finite amount of work

12. Deadlocks • Example thread A thread B lock(x) lock(y) lock(y) lock(x) ... ... unlock(y) unlock(x) unlock(x) unlock(y)

13. Deadlocks • Can deadlock occur with code? • Will deadlock always occur with this code?

14. Dining Philosophers Problem

15. Dining Philosophers • 5 philosophers sitting around a round table • 1 chopstick in between each pair of philosophers • 5 chopsticks total • Each philosopher needs two chopsticks to eat • How to prevent deadlock

16. Dining Philosophers • Algorithm for each philosopher • wait for chopstick on right to be free, then pick it up • wait for chopstick on left to be free, then pick it up • eat • put both chopsticks down • Can this deadlock?

17. Conditions for Deadlock • 4 conditions must all be true for deadlock to occur • Limited resource: • not enough resources to serve all threads simultaneously • Hold and wait: • threads hold resources while waiting to acquire others

18. Thread A Thread B Conditions for Deadlock • No preemption • can’t force thread to give up resource • Circular chain of requests Resource A Resource B

19. Four Conditions for Deadlock • The first condition is sometimes called • Mutual exclusion condition • each resource assigned to 1 process or is available • Tanebaum, etc.

20. Deadlock Modeling • Modeled with directed graphs

21. Deadlock Modeling • Resource R assigned to process A • Process B is requesting/waiting for resource S • Process C and D are in deadlock • over resources T and U

22. Deadlock Modeling

23. How deadlock can be avoided (o) (p) (q)

24. Strategies for dealing with Deadlocks • Ignore: just ignore the problem altogether • Detection and recovery • Dynamic avoidance: careful resource allocation • Prevention: • negating one of the four necessary conditions

25. Ignore Strategy • Pretend there is no problem • Reasonable if • deadlocks occur very rarely • cost of prevention is high • UNIX and Windows takes this approach • a trade off between convenience & correctness

26. Detect and Fix • Note the resource ownership and requests • Detect by looking for cycles in the wait-for graph • How to fix once detected?

27. Detection with Wait-for Graph

28. Detection with Wait-for Graph

29. Detection with Wait-for Graph An example for the deadlock detection algorithm

30. Recovery from Deadlock • Recovery through preemption • take a resource from some other process • depends on nature of the resource • Recovery through rollback • checkpoint a process periodically • use this saved state • restart the process if it is found deadlocked

31. Recovery from Deadlock • Recovery through killing processes • crudest but simplest way to break a deadlock • kill one of the processes in the deadlock cycle • the other processes get its resources • choose process that can be rerun from the beginning

32. Deadlock Avoidance • Don’t get in a position where • Deadlock becomes possible • Use: • Resource Trajectories

33. Safe and Unsafe States • Demonstration that the state in (a) is safe Has Max Has Max Has Max Has Max (a) Free: 3 (b) Free: 1 (c) Free: 5 (d) Free: 0

34. Safe and Unsafe States • Demonstration that the state in b is not safe Has Max Has Max Has Max Has Max (a) Free: 3 (b) Free: 2 (c) Free: 0 (d) Free: 4

35. Deadlock Prevention • Idea is to eliminate one of the four necessary conditions • Increase resources to decrease waiting • this minimizes chance of deadlock

36. Deadlock Prevention • Attacking the Mutual Exclusion Condition • Attacking the Hold and Wait Condition • Attacking the No Preemption Condition • Attacking the Circular Wait Condition

37. Attacking Mutual Exclusion • Some devices (such as printer) can be spooled • only the printer daemon uses printer resource • thus deadlock for printer eliminated • Not all devices can be spooled

38. Attacking Mutual Exclusion • Principle: • Avoid assigning resource when not absolutely necessary • As few processes as possible actually claim the resource

39. Attacking Hold & Wait Condition • Processes to request resources before starting • a process never has to wait for what it needs • Eliminate hold and wait

40. Attacking Hold & Wait Condition • phase 1a. acquire all resources • phase 1b. while (not done) { • acquire some resource • work • } • phase 2. release all resources

41. Attacking Hold & Wait Condition • A. wait until all resources you’ll need are free, then grab them all at once • (or) B. if you find resource busy, release all acquired resources and go back to beginning

42. Attacking Hold & Wait Condition • Problems? • May not know required resources at start of run • Ties up resources others could be using

43. Attacking Hold & Wait Condition • Variation: • Process must give up all resources • Then request all immediately needed

44. Attacking No Preemption Condition • Allow Preemption! • Can preempt CPU by saving its state to thread control block and resuming later • Can preempt memory by swapping memory out to disk and loading it back later • Can we preempt the holding of a lock?

45. Attacking No Preemption Condition • Some resource cannot be preempted • Consider a process given the printer • halfway through its job • now forcibly take away printer • !!??

46. Attacking Circular Wait Condition • Normally ordered resources • Imagesetter  scanner  plotter  printer • A resource graph

47. Banker’s Algorithm • Derive from methods by bankers to grant loans • Similar to reserving all resources at beginning • but more efficient

48. Banker’s Algorithm • State maximum resource needs in advance • but don’t actually acquire the resources • When thread later tries to acquire a resource, Banker’s algorithm determines: • when it’s safe to satisfy the request • and blocks the thread when it’s not safe

49. Banker’s Algorithm • General structure of thread code • phase 1a. state maximum resource needed • phase 1b. while (not done) { • acquire some resources • work • } • phase 2. release all resources

50. Banker’s Algorithm • Preventing deadlock by requesting all resources at beginning would block thread in phase 1a above • but phase 1b can proceed without waiting