Chapter 7 Deadlocks Joe Mc Carthy CSS 430

Chapter 7: Deadlocks Joe Mc. Carthy CSS 430: Operating Systems - Deadlocks 1

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 CSS 430: Operating Systems - Deadlocks 2

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 wait (A); wait (B); P 1 wait(B) wait(A) CSS 430: Operating Systems - Deadlocks 3

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

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

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

Java Deadlock Example class Named. Reentrant. Lock extends Reentrant. Lock { private String name; public Named. Reentrant. Lock( String name ) { this. name = name; } public String to. String() { return String. format( "Lock-%s", name ); } } public class Deadlock. Example { public static void main( String arg[] ) { Lock lock 1 = new Named. Reentrant. Lock( "1" ); Lock lock 2 = new Named. Reentrant. Lock( "2" ); Thread thread. A = new Thread( new Deadlockable( "A", lock 1, lock 2 ) ); Thread thread. B = new Thread( new Deadlockable( "B", lock 2, lock 1 ) ); thread. A. start(); thread. B. start(); } } CSS 430: Operating Systems - Deadlocks 7

Java Deadlock Example $ java Thread Thread Deadlock. Example A wants to acquire A has now acquired B wants to acquire B has now acquired A wants to acquire B wants to acquire lock lock Lock-1 Lock-2 Lock-1 ~css 432/examples/os-book/ch 7 CSS 430: Operating Systems - Deadlocks 8

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

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

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 - Deadlocks 11

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, …, Pn}, the set consisting of all the processes in the system – R = {R 1, R 2, …, Rm}, the set consisting of all resource types in the system • request edge – directed edge Pi Rj • assignment edge – directed edge Rj Pi CSS 430: Operating Systems - Deadlocks 12

Resource Allocation Graph • Process P • Resource R with 4 instances • Pi requests an instance of Rj Pi Rj • Pi is holding an instance of Rj Pi Rj • Pi releases an instance of Rj Pi Rj CSS 430: Operating Systems - Deadlocks 13

Resource Allocation Graph CSS 430: Operating Systems - Deadlocks 14

Resource Allocation Graph CSS 430: Operating Systems - Deadlocks 15

Resource Allocation Graph CSS 430: Operating Systems - Deadlocks 16

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

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

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 CSS 430: Operating Systems - Deadlocks 19

Deadlock Prevention CSS 430: Operating Systems - Deadlocks 20

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

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 - Deadlocks 22

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 - Deadlocks 23

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 resourceallocation state to ensure no circular wait CSS 430: Operating Systems - Deadlocks 24

Safe State • Safe state: system can allocate resources to each process in some order and not enter a deadlocked state • Safe sequence: an ordering <P 1, P 2, …, Pn> of ALL processes such that – for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j < i • That is: – If Pi needs resources not immediately available, then Pi can wait until P 1 … Pj have finished. – When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate. – When Pi terminates, Pi +1 can obtain its needed resources, … CSS 430: Operating Systems - Deadlocks 25

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 - Deadlocks 26

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 - Deadlocks 27

Resource Allocation Graph Scheme • Claim edge Pi Rj indicated that process Pj may request resource Rj; 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 - Deadlocks 28

Resource Allocation Graph CSS 430: Operating Systems - Deadlocks 29

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

Resource Allocation Graph Algorithm • Suppose that process Pi requests a resource Rj • 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 - Deadlocks 31

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 * Named in the pre too-big-to-fail era CSS 430: Operating Systems - Deadlocks 32

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 Rj available • Max: n x m matrix. If Max[i, j] = k, then process Pi may request at most k instances of resource type Rj • Allocation: n x m matrix. If Allocation[i, j] = k then Pi is currently allocated k instances of Rj • Need: n x m matrix. If Need[i, j] = k, then Pi may need k more instances of Rj to complete its task Need [i, j] = Max[i, j] – Allocation[i, j] CSS 430: Operating Systems - Deadlocks 33

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

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

Example of Banker’s Algorithm • 5 processes P 0 through P 4; 3 resource types: A (10 instances), B (5 instances), and C (7 instances) Snapshot at time T 0: Allocation Max Available ABC ABC P 0 0 1 0 753 332 P 1 2 0 0 322 P 2 3 0 2 902 P 3 2 1 1 222 P 4 0 0 2 433 CSS 430: Operating Systems - Deadlocks 36

Example of Banker’s Algorithm • The content of the matrix Need is defined to be Max – Allocation Need ABC 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 < P 1, P 3, P 4, P 2, P 0> satisfies safety criteria CSS 430: Operating Systems - Deadlocks 37

Example: P 1 Request (1, 0, 2) • Check that Request Available (that is, (1, 0, 2) (3, 3, 2) true Allocation Need Available ABC ABC P 0 0 1 0 743 230 P 1 3 0 2 020 P 2 3 0 1 600 P 3 2 1 1 011 P 4 0 0 2 431 • Executing safety algorithm shows that sequence < P 1, P 3, P 4, P 0, P 2> satisfies safety requirement CSS 430: Operating Systems - Deadlocks 38

Example: P 1 Request (1, 0, 2) • Check that Request Available (that is, (1, 0, 2) (3, 3, 2) true Allocation Need Available ABC ABC P 0 0 1 0 743 230 P 1 3 0 2 020 P 2 3 0 1 600 P 3 2 1 1 011 P 4 0 0 2 431 • Executing safety algorithm shows that sequence < P 1, P 3, P 4, P 0, P 2> 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 - Deadlocks 39

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

Single Instance of Each Resource Type • Maintain wait-for graph – Nodes are processes – Pi Pj if Pi is waiting for Pj • 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 - Deadlocks 41

Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph CSS 430: Operating Systems - Deadlocks 42

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 [ij] = k, then process Pi is requesting k more instances of resource type. Rj. CSS 430: Operating Systems - Deadlocks 43

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 Allocationi 0, then Finish[i] = false; otherwise, Finish[i] = true 2. Find an index i such that both: (a) Finish[i] == false (b) Requesti Work If no such i exists, go to step 4 3. Work = Work + Allocationi 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 Pi is deadlocked Algorithm requires an order of O(m x n 2) operations CSS 430: Operating Systems - Deadlocks 44

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 Request Available ABC ABC P 0 0 1 0 000 P 1 2 0 0 202 P 2 3 000 P 3 2 1 1 100 P 4 0 0 2 002 • Sequence <P 0, P 2, P 3, P 1, P 4> will result in Finish[i] = true for all i CSS 430: Operating Systems - Deadlocks 45

Example of Detection Algorithm • P 2 requests an additional instance of type C Request ABC P 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? CSS 430: Operating Systems - Deadlocks 46

Example of Detection Algorithm • P 2 requests an additional instance of type C Request ABC P 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 CSS 430: Operating Systems - Deadlocks 47

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 - Deadlocks 48

Recovery from Deadlock: • Deadlock detected … now what? CSS 430: Operating Systems - Deadlocks 49

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 - Deadlocks 50

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 - Deadlocks 51

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 - Deadlocks 52