1. Chapter 7: Deadlocks Joe McCarthy CSS 430: Operating Systems - Deadlocks1

2. Chapter 7: Deadlocks The Deadlock Problem System Model Deadlock Characterization Deadlock Example Approaches to Handling Deadlocks Material derived, in part, from Operating Systems Concepts with Java, 8 th Ed. © 2009 Silberschatz, Galvin & Gagne 2CSS 430: Operating Systems - Deadlocks

3. The Deadlock Problem A set of blocked processes each holding a resource and waiting to acquire another resource held by another process in the set Example – System has 2 disk drives – P 1 and P 2 each hold one disk drive and each needs another one Example semaphores A and B, initialized to 1 P 0 P 1 wait (A);wait(B) wait (B);wait(A) CSS 430: Operating Systems - Deadlocks3

4. System Model Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices May have multiple instances of R i Resource utilization sequence: 1.request 2.use (critical section) 3.release 4CSS 430: Operating Systems - Deadlocks

5. Deadlock Characterization Deadlock can arise if four conditions hold simultaneously: 1.Mutual exclusion: only one process at a time can use a resource 2.Hold and wait: a process holding at least one resource is waiting to acquire other resources held by other processes 3.No preemption: a resource can only be released voluntarily by the process holding it, after that process has completed its task. 4.Circular wait: there exists a set {P 0, P 1, …, P n } of waiting processes such that P 0 is waiting for a resource that is held by P 1 P 1 is waiting for a resource that is held by P 2 … P n is waiting for a resource that is held by P 0 5CSS 430: Operating Systems - Deadlocks

6. Java Deadlock Example 6CSS 430: Operating Systems - Deadlocks class Deadlockable implements Runnable { private String name; private Lock first, second; public Deadlockable( String name, Lock first, Lock second ) { this.name = name; this.first = first; this.second = second; } public void run() { try { System.out.printf( "Thread %s wants to acquire lock %s\n", name, first ); first.lock(); System.out.printf( "Thread %s has now acquired lock %s\n", name, first ); SleepUtilities.nap( 3 ); System.out.printf( "Thread %s wants to acquire lock %s\n", name, second ); second.lock(); System.out.printf( "Thread %s has now acquired lock %s\n", name, second ); } finally { first.unlock(); second.unlock(); }

7. Java Deadlock Example 7CSS 430: Operating Systems - Deadlocks class NamedReentrantLock extends ReentrantLock { private String name; public NamedReentrantLock( String name ) { this.name = name; } public String toString() { return String.format( "Lock-%s", name ); } public class DeadlockExample { public static void main( String arg[] ) { Lock lock1 = new NamedReentrantLock( "1" ); Lock lock2 = new NamedReentrantLock( "2" ); Thread threadA = new Thread( new Deadlockable( "A", lock1, lock2 ) ); Thread threadB = new Thread( new Deadlockable( "B", lock2, lock1 ) ); threadA.start(); threadB.start(); }

8. Java Deadlock Example 8CSS 430: Operating Systems - Deadlocks $ java DeadlockExample Thread A wants to acquire lock Lock-1 Thread A has now acquired lock Lock-1 Thread B wants to acquire lock Lock-2 Thread B has now acquired lock Lock-2 Thread A wants to acquire lock Lock-2 Thread B wants to acquire lock Lock-1 ~css432/examples/os-book/ch7

9. Approaches to Handling Deadlocks Deadlock conditions: CSS 430: Operating Systems - Deadlocks9

10. Approaches to Handling Deadlocks Deadlock conditions: – Mutual Exclusion – Hold & Wait – No preemption – Circular Wait Handling Deadlocks: CSS 430: Operating Systems - Deadlocks10

11. Approaches to Handling Deadlocks Deadlock conditions: – Mutual Exclusion – Hold & Wait – No preemption – Circular Wait Handling Deadlocks: – Prevent or avoid deadlocks – Detect deadlock states & then recover – Ignore deadlocks Used by most OSs, including Linux & Windows (also JVM) CSS 430: Operating Systems - Deadlocks11

12. Resource Allocation Graph A set of vertices (V) and a set of edges (E) V is partitioned into two types: – P = {P 1, P 2, …, P n }, the set consisting of all the processes in the system – R = {R 1, R 2, …, R m }, the set consisting of all resource types in the system request edge – directed edge P i  R j assignment edge – directed edge R j  P i 12CSS 430: Operating Systems - Deadlocks

13. Resource Allocation Graph Process P Resource R with 4 instances P i requests an instance of R j P i is holding an instance of R j P i releases an instance of R j CSS 430: Operating Systems - Deadlocks13 PiPi PiPi RjRj RjRj PiPi RjRj

14. Resource Allocation Graph CSS 430: Operating Systems - Deadlocks14

15. Resource Allocation Graph CSS 430: Operating Systems - Deadlocks15

16. Resource Allocation Graph CSS 430: Operating Systems - Deadlocks16

17. Cycle  Deadlock? CSS 430: Operating Systems - Deadlocks17 Four conditions Mutual Exclusion Hold & Wait No preemption Circular Wait

18. Cycle  Deadlock? CSS 430: Operating Systems - Deadlocks18 Four conditions Mutual Exclusion Hold & Wait No preemption Circular Wait

19. Basic Facts If graph contains no cycles  no deadlock If graph contains a cycle  – if only one instance per resource type, then deadlock – if several instances per resource type, possibility of deadlock 19CSS 430: Operating Systems - Deadlocks

20. Deadlock Prevention CSS 430: Operating Systems - Deadlocks20

21. Deadlock Prevention Eliminate one of the four conditions – Mutual exclusion – Hold and Wait – No preemption – Circular Wait CSS 430: Operating Systems - Deadlocks21

22. Deadlock Prevention Mutual Exclusion: – OK for sharable resources (e.g., read-only files) – Not OK for nonsharable resources Hold and Wait: – Requests allowed only if process is holding none Request & be allocated all or nothing Release current Rs before requesting new Rs – Low resource utilization; starvation possible CSS 430: Operating Systems - Deadlocks22

23. Deadlock Prevention No Preemption: – If a process that is holding some resources requests another resource that cannot be immediately allocated, then all resources currently being held are released – Preempted resources are added to the list of resources for which the process is waiting – Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting Circular Wait: – impose a total ordering of all resource types – Require that resources are requested in order CSS 430: Operating Systems - Deadlocks23

24. Deadlock Avoidance Requires a priori information – Require that each process declare the maximum number of resources of each type that it may need Resource-allocation state: – number of available and allocated resources – maximum demands of the processes Algorithm: dynamically examine the resource- allocation state to ensure no circular wait CSS 430: Operating Systems - Deadlocks24

25. Safe State Safe state: system can allocate resources to each process in some order and not enter a deadlocked state Safe sequence: an ordering of ALL processes such that – for each P i, the resources that P i can still request can be satisfied by currently available resources + resources held by all the P j, with j < i That is: – If P i needs resources not immediately available, then P i can wait until P 1 … P j have finished. – When P j is finished, P i can obtain needed resources, execute, return allocated resources, and terminate. – When P i terminates, P i +1 can obtain its needed resources, … CSS 430: Operating Systems - Deadlocks25

26. Safe, Unsafe & Deadlock states If system is in safe state  no deadlocks If system is in unsafe state  possibility of deadlock Avoidance: ensure that a system will never enter an unsafe state CSS 430: Operating Systems - Deadlocks26

27. Avoidance algorithms Single instance of a resource type – Use a resource-allocation graph Multiple instances of a resource type – Use the Banker’s Algorithm CSS 430: Operating Systems - Deadlocks27

28. Resource Allocation Graph Scheme Claim edge P i  R j indicated that process P j may request resource R j ; represented by a dashed line Claim edge converts to request edge when a process requests a resource Request edge converted to an assignment edge when the resource is allocated to the process When a resource is released by a process, assignment edge reconverts to a claim edge Resources must be claimed a priori in the system CSS 430: Operating Systems - Deadlocks28

29. Resource Allocation Graph CSS 430: Operating Systems - Deadlocks29

30. Unsafe State in Resource Allocation Graph CSS 430: Operating Systems - Deadlocks30

31. Resource Allocation Graph Algorithm Suppose that process P i requests a resource R j The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph CSS 430: Operating Systems - Deadlocks31

32. Banker’s Algorithm Resource allocation & deadlock avoidance Idea: bank should never allocate its money in such a way that it can no longer satisfy the needs of all its customers* Assumptions: – Multiple instances of resource types – Each process must a priori claim maximum use – When a process requests a resource it may have to wait – When a process gets all its resources it must return them in a finite amount of time CSS 430: Operating Systems - Deadlocks32 * Named in the pre too-big-to-fail era

33. Banker’s Algorithm: Data Structures Let n = # of processes, m = # of resources types Available: Vector of length m. If Available[j] = k, there are k instances of resource type R j available Max: n x m matrix. If Max[i,j] = k, then process P i may request at most k instances of resource type R j Allocation: n x m matrix. If Allocation[i,j] = k then P i is currently allocated k instances of R j Need: n x m matrix. If Need[i,j] = k, then P i may need k more instances of R j to complete its task Need [i,j] = Max[i,j] – Allocation[i,j] CSS 430: Operating Systems - Deadlocks33

34. Safety Algorithm 1.Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish[i] = false, for i = 0.. n-1 2.Find an i such that both: Finish[i] == false Need[i]  Work If no such i exists, go to step 4 3.Update: Work = Work + Allocation[i] Finish[i] = true go to step 2 4.If Finish [i] == true for all i, then the system is in a safe state CSS 430: Operating Systems - Deadlocks34

35. Resource-Request Algorithm Request = request vector for process P i. If Request i [j] = k then process P i wants k instances of resource type R j 1.If Request i  Need i go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim 2.If Request i  Available, go to step 3. Otherwise P i must wait, since resources are not available 3.Pretend to allocate requested resources to P i by modifying the state as follows: Available = Available – Request; Allocation i = Allocation i + Request i ; Need i = Need i – Request i ; 4.If safe  the resources are allocated to P i If unsafe  P i must wait, and the old resource-allocation state is restored CSS 430: Operating Systems - Deadlocks35

36. Example of Banker’s Algorithm 5 processes P 0 through P 4 ; 3 resource types: A (10 instances), B (5instances), and C (7 instances) Snapshot at time T 0 : Allocation MaxAvailable A B C A B C A B C P 0 0 1 0 7 5 3 3 3 2 P 1 2 0 0 3 2 2 P 2 3 0 2 9 0 2 P 3 2 1 1 2 2 2 P 4 0 0 2 4 3 3 CSS 430: Operating Systems - Deadlocks36

37. Example of Banker’s Algorithm The content of the matrix Need is defined to be Max – Allocation Need A B C P 0 7 4 3 P 1 1 2 2 P 2 6 0 0 P 3 0 1 1 P 4 4 3 1 The system is in a safe state since the sequence satisfies safety criteria CSS 430: Operating Systems - Deadlocks37

38. Example: P 1 Request (1,0,2) Check that Request  Available (that is, (1,0,2)  (3,3,2)  true AllocationNeedAvailable A B C A B CA B C P 0 0 1 0 7 4 3 2 3 0 P 1 3 0 2 0 2 0 P 2 3 0 1 6 0 0 P 3 2 1 1 0 1 1 P 4 0 0 2 4 3 1 Executing safety algorithm shows that sequence satisfies safety requirement CSS 430: Operating Systems - Deadlocks38

39. Example: P 1 Request (1,0,2) Check that Request  Available (that is, (1,0,2)  (3,3,2)  true AllocationNeedAvailable A B C A B CA B C P 0 0 1 0 7 4 3 2 3 0 P 1 3 0 2 0 2 0 P 2 3 0 1 6 0 0 P 3 2 1 1 0 1 1 P 4 0 0 2 4 3 1 Executing safety algorithm shows that sequence satisfies safety requirement Can request for (3,3,0) by P 4 be granted? Can request for (0,2,0) by P 0 be granted? CSS 430: Operating Systems - Deadlocks39

40. Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme CSS 430: Operating Systems - Deadlocks40

41. Single Instance of Each Resource Type Maintain wait-for graph – Nodes are processes – P i  P j if P i is waiting for P j Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock An algorithm to detect a cycle in a graph requires an order of n 2 operations, where n is the number of vertices in the graph CSS 430: Operating Systems - Deadlocks41

42. Resource-Allocation Graph and Wait-for Graph CSS 430: Operating Systems - Deadlocks42 Resource-Allocation GraphCorresponding wait-for graph

43. Multiple Resource Instances of a Resource Type Available: A vector of length m indicates the number of available resources of each type. Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. Request: An n x m matrix indicates the current request of each process. If Request [i j ] = k, then process P i is requesting k more instances of resource type. R j. CSS 430: Operating Systems - Deadlocks43

44. Detection Algorithm 1.Let Work and Finish be vectors of length m and n, respectively Initialize: (a) Work = Available (b) For i = 1,2, …, n, if Allocation i  0, then Finish[i] = false; otherwise, Finish[i] = true 2.Find an index i such that both: (a)Finish[i] == false (b)Request i  Work If no such i exists, go to step 4 3.Work = Work + Allocation i Finish[i] = true go to step 2 4.If Finish[i] == false, for some i, 1  i  n, then the system is in deadlock state. Moreover, if Finish[i] == false, then P i is deadlocked 44CSS 430: Operating Systems - Deadlocks Algorithm requires an order of O(m x n 2) operations

45. Example of Detection Algorithm Five processes P 0 through P 4 ; three resource types A (7 instances), B (2 instances), and C (6 instances) Snapshot at time T 0 : Allocation RequestAvailable A B C A B C A B C P 0 0 1 0 0 0 0 0 0 0 P 1 2 0 0 2 0 2 P 2 3 0 3 0 0 0 P 3 2 1 1 1 0 0 P 4 0 0 2 0 0 2 Sequence will result in Finish[i] = true for all i CSS 430: Operating Systems - Deadlocks45

46. Example of Detection Algorithm P 2 requests an additional instance of type C Request A B C P 0 0 0 0 P 1 2 0 1 P 2 0 0 1 P 3 1 0 0 P 4 0 0 2 State of system? 46CSS 430: Operating Systems - Deadlocks

47. Example of Detection Algorithm P 2 requests an additional instance of type C Request A B C P 0 0 0 0 P 1 2 0 1 P 2 0 0 1 P 3 1 0 0 P 4 0 0 2 State of system: – Can reclaim resources held by process P 0, but insufficient resources to fulfill other processes; requests – Deadlock exists, consisting of processes P 1, P 2, P 3, and P 4 47CSS 430: Operating Systems - Deadlocks

48. Detection-Algorithm Usage When, and how often, to invoke depends on: – How often a deadlock is likely to occur? – How many processes will need to be rolled back? one for each disjoint cycle If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes “caused” the deadlock. CSS 430: Operating Systems - Deadlocks48

49. Recovery from Deadlock: Deadlock detected … now what? CSS 430: Operating Systems - Deadlocks49

50. Recovery from Deadlock: Deadlock detected … now what? Recall the of the four conditions – Mutual exclusion – Hold and Wait – No preemption – Circular Wait CSS 430: Operating Systems - Deadlocks50

51. Recovery from Deadlock: Process Termination Which one(s)? – Abort all deadlocked processes – Abort one process at a time until the deadlock cycle is eliminated In what order? – Priority of the process – Time process has computed, time to completion – Resources process has used, resources needed – How many processes will need to be terminated? – Is process interactive or batch? CSS 430: Operating Systems - Deadlocks51

52. Recovery from Deadlock: Resource Preemption Selecting a victim – minimize cost Rollback – return to some safe state, restart process for that state Starvation – same process may always be picked as victim, include number of rollback in cost factor CSS 430: Operating Systems - Deadlocks52