Chapter 7 Deadlocks n The Deadlock Problem n

Chapter 7: Deadlocks n The Deadlock Problem n System Model n Deadlock Characterization n Methods for Handling Deadlocks n Deadlock Prevention n Deadlock Avoidance n Deadlock Detection n Recovery from Deadlock Operating System Principles 7. 1 Silberschatz, Galvin and Gagne © 2005

Chapter Objectives n To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks n To present a number of different methods for preventing or avoiding deadlocks in a computer system. Operating System Principles 7. 2 Silberschatz, Galvin and Gagne © 2005

The Deadlock Problem n Definition: A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set. n Example l System has 2 disk drives. l P 1 and P 2 each hold one disk drive and each needs another one. n Example l semaphores A and B, initialized to 1 P 0 wait (A); wait (B); signal(A); signal(B); Operating System Principles P 1 wait(B); wait(A); signal(B); signal(A); 7. 3 Silberschatz, Galvin and Gagne © 2005

Bridge Crossing Example n Traffic only in one direction. n Each section of a bridge can be viewed as a resource. n If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback). n Several cars may have to be backed up if a deadlock occurs. n Starvation is possible. Operating System Principles 7. 4 Silberschatz, Galvin and Gagne © 2005

7. 1 System Model n Resource types R 1, R 2, . . . , Rm CPU cycles, memory space, I/O devices, files, semaphores n Each resource type Ri has Wi instances. l If a process request an instance of a resource type, the allocation of any instance of the type will satisfy the request n Each process utilizes a resource as follows: 1. Request: open( ), allocate( ), wait( ) 2. use 3. Release: close( ), free( ), signal( ) Operating System Principles 7. 5 Silberschatz, Galvin and Gagne © 2005

Deadlock in Multi-thread Applications n Multiple threads can compete for shared resources pthread_mutex_t first_mutex; pthread_mutex_t second_mutex; pthread_mutex_init(&first_mutex, NULL); pthread_mutex_init(&second_mutex, NULL); initialization void *do_work_two(void *param) { pthread_mutex_lock(&second_mutex); pthread_mutex_lock(&first_mutex); void *do_work_one(void *param) { pthread_mutex_lock(&first_mutex); pthread_mutex_lock(&second_mutex); /* do some work */ pthread_mutex_unlock(&second_mutex); pthread_mutex_unlock(&first_mutex); pthread_mutex_unlock(&second_mutex); pthread_exit(0); } } thread_two thread_one Operating System Principles 7. 6 Silberschatz, Galvin and Gagne © 2005

7. 2 Deadlock Characterization Deadlock can arise if four necessary conditions hold simultaneously: n Mutual exclusion: only one process at a time can use a resource. n Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes. n No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task. n 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, P 1 is waiting for a resource that is held by P 2, …, Pn – 1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P 0. Operating System Principles 7. 7 Silberschatz, Galvin and Gagne © 2005

Resource-Allocation Graph It is a set of vertices V and a set of edges E. n V is partitioned into two types: l P = {P 1, P 2, …, Pn}, the set consisting of all the processes in the system. l R = {R 1, R 2, …, Rm}, the set consisting of all resource types in the system. Each instance is represented as a dot inside the rectangle. ‧‧ ‧‧ n request edge – directed edge Pi Rj n assignment edge – directed edge Rj Pi, and one of the dots in the rectangle must be designated Operating System Principles 7. 8 Silberschatz, Galvin and Gagne © 2005

Resource-Allocation Graph (Cont. ) Example: n Process n Resource Type with 4 instances ‧‧ ‧‧ n Pi requests instance of Rj ‧‧ ‧‧ Pi Rj n Pi is holding an instance of Rj Pi simplification assumption: no process will request the same resource more than one instance ‧‧ ‧‧ Rj Operating System Principles 7. 9 Silberschatz, Galvin and Gagne © 2005

Resource Allocation Graph With A Cycle but No Deadlock Resource Allocation Graph With A Deadlock Operating System Principles 7. 10 Silberschatz, Galvin and Gagne © 2005

Basic Facts n If graph contains no cycles no deadlock. n If graph contains a cycle l if only one instance per resource type, then deadlock. l if several instances per resource type, possibility of deadlock. Operating System Principles 7. 11 Silberschatz, Galvin and Gagne © 2005

7. 3 Methods for Handling Deadlocks 1. Prevent or avoid deadlocks, ensuring that the system will never enter a deadlock state. 2. Allow the system to enter a deadlock state, detect it and then recover. 3. Ignore the problem and pretend that deadlocks never occur in the system n used by most operating systems, including UNIX. Operating System Principles 7. 12 Silberschatz, Galvin and Gagne © 2005

7. 4 Deadlock Prevention Restrain the ways request can be made. n Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources (like printer). l Since some resources are intrinsically nonsharable, this condition cannot be applied in general. n Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. l Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none. l Low resource utilization; starvation possible. Operating System Principles 7. 13 Silberschatz, Galvin and Gagne © 2005

Deadlock Prevention (Cont. ) n No Preemption – l If a process that is holding some resources requests another resource that cannot be immediately allocated to it, then all resources currently being held by this process are released. l Preempted resources are added to the list of resources for which the process is waiting. l Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. l This is applied to resources whose state can be easily saved and restored later, such as CPU registers and memory space. Operating System Principles 7. 14 Silberschatz, Galvin and Gagne © 2005

Deadlock Prevention (Cont. ) n Circular Wait – impose a total ordering of all resource types l require that each process requests resources in an increasing order of enumeration. 4 If several instances of a resource type are needed, a single request for all of them must be made Alternatively, we can require that whenever a process requests an instance of resource type Rj, it must have released any resources Ri with a higher number. l In an application program, this could be accomplished by developing an order among all synchronization objects, like semaphores or locks, in the system. l It is up to the application developers to write programs that follow the ordering l In Free. BSD, the witness lock-order verifier dynamically maintaining the relationship of lock orders in the system. l 4 See example in page 294 Operating System Principles 7. 15 Silberschatz, Galvin and Gagne © 2005

7. 5 Deadlock Avoidance Requires that the system has some additional a priori (事先) information available. n Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need. n The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. n Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes. Operating System Principles 7. 16 Silberschatz, Galvin and Gagne © 2005

Safe State n When a process requests an available resource, system must decide if immediate allocation still keeps the system in a safe state. l It can allocate resources to each process (up to its maximum) in some order and still avoid a deadlock Operating System Principles 7. 17 Silberschatz, Galvin and Gagne © 2005

Safe State n System is in safe state if there exists a sequence <P 1, P 2, …, Pn> of ALL the processes in the systems 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. l If Pi resource needs are not immediately available, then Pi can wait until all Pj with j < i have finished. l When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate. l When Pi terminates, Pi +1 can obtain its needed resources, and so on. Operating System Principles 7. 18 Silberschatz, Galvin and Gagne © 2005

Basic Facts n If a system is in safe state no deadlocks. n If a system is in unsafe state possibility of deadlock. n Avoidance ensure that a system will never enter an unsafe state. n If a process requests a resource that is currently available, it may still have to wait Maximum Needs Current Holds P 0 10 5 P 1 4 2 P 2 9 2 Operating System Principles 7. 19 Silberschatz, Galvin and Gagne © 2005

7. 5. 2 Avoidance algorithms n Resources must be claimed a priori in the system. l Claim edge Pi Rj indicated that process Pj may request resource Rj; represented by a dashed line. l Claim edge converts to request edge when a process requests a resource. l Request edge converted to an assignment edge when the resource is allocated to the process. l When a resource is released by a process, assignment edge reconverts to a claim edge. n Single instance of a resource type. l Use a resource-allocation graph. n Multiple instances of a resource type. l Use the banker’s algorithm. Operating System Principles 7. 20 Silberschatz, Galvin and Gagne © 2005

Resource-Allocation Graph Algorithm n Suppose that process Pi requests a resource Rj n 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 l If no cycle exists, then the allocation of the resource will keep the system in a safe state l If a cycle is found, process Pi will have to wait for its requests to be satisfied Operating System Principles 7. 21 Silberschatz, Galvin and Gagne © 2005

Resource-Allocation Graph Scheme Resource-Allocation Graph Operating System Principles 7. 22 Unsafe State In Resource-Allocation Graph Silberschatz, Galvin and Gagne © 2005

7. 5. 3 Banker’s Algorithm n The algorithm could be used in a banking system to ensure that the bank never allocated its available cash such that it could no longer satisfy the needs of all customers n Multiple instances of some resource types. n Each process must a priori claim maximum use. n When a process requests a resource it may have to wait. n When a process gets all its resources it must return them in a finite amount of time. Operating System Principles 7. 23 Silberschatz, Galvin and Gagne © 2005

Data Structures for the Banker’s Algorithm Let n = number of processes, and m = number of resources types. n Available: Vector of length m. If available [j] = k, there are k instances of resource type Rj available. n Max: n x m matrix. If Max [i, j] = k, then process Pi may request at most k instances of resource type Rj. n Allocation: n x m matrix. If Allocation[i, j] = k then Pi is currently allocated k instances of Rj. n 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]. Operating System Principles 7. 24 Silberschatz, Galvin and Gagne © 2005

Safety Algorithm 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available // number of available instances of each resource type Finish [i] = false for i = 0, 1, …, n- 1. 2. Find an i such that both: (a) Finish [i] = false (b) Needi Work (pairwise comparison for all resource types) If no such i exists, go to step 4. 3. Work = Work + Allocationi Finish[i] = true go to step 2. 4. If Finish [i] == true for all i, then the system is in a safe state. Otherwise, it is unsafe. Operating System Principles 7. 25 Silberschatz, Galvin and Gagne © 2005

Resource-Request Algorithm for Requests Requesti = request vector for process Pi. Requesti [j] = k : process Pi wants k instances of resource type Rj. Actions taken when Requesti is made by process Pi: 1. 2. 3. 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 – Requesti; Allocationi = Allocationi + Requesti; Needi = Needi – Requesti; l If resulting state safe the resources are allocated to Pi. l If unsafe Pi must wait, and the old resource-allocation state is restored Operating System Principles 7. 26 Silberschatz, Galvin and Gagne © 2005

Example of Banker’s Algorithm n 5 processes: P 0 through P 4; n 3 resource types: A (10 instances), B (5 instances), and C (7 instances). n Snapshot at time T 0: Operating System Principles Allocation Max Available ABC ABC P 0 010 753 332 P 1 200 322 P 2 302 902 P 3 211 222 P 4 002 433 7. 27 Silberschatz, Galvin and Gagne © 2005

Example (Cont. ) n The content of the matrix Need is defined to be Max – Allocation. Need ABC P 0 743 P 1 122 P 2 600 P 3 011 P 4 431 n The system is in a safe state since the sequence < P 1, P 3, P 4, P 2, P 0> satisfies safety criteria. Operating System Principles 7. 28 Silberschatz, Galvin and Gagne © 2005

Example: P 1 Request (1, 0, 2) n Check whether Request Available? (that is, (1, 0, 2) (3, 3, 2) ? ) l Allocation Need Available ABC ABC P 0 010 743 230 P 1 302 020 P 2 301 600 P 3 211 011 P 4 002 431 Executing safety algorithm shows that sequence < P 1, P 3, P 4, P 0, P 2> satisfies safety requirement. We can immediately grant the request. n Can request for (3, 3, 0) by P 4 be granted? n Can request for (0, 2, 0) by P 0 be granted? Operating System Principles 7. 29 Silberschatz, Galvin and Gagne © 2005

7. 6 Deadlock Detection n Allow system to enter deadlock state n Detection algorithm: an algorithm that examines the state of the system to determine whether a deadlock has occurred n Recovery scheme: an algorithm to recover from the deadlock. This has potential loss in the recovering. Operating System Principles 7. 30 Silberschatz, Galvin and Gagne © 2005

7. 6. 1 Single Instance of Each Resource Type n Maintain wait-for graph l Nodes are processes. l Pi Pj if Pi is waiting for Pj to release a resource that Pi needs n Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock. n 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. Operating System Principles 7. 31 Silberschatz, Galvin and Gagne © 2005

Resource-Allocation Graph and Wait-for Graph Corresponding wait-for graph Resource-Allocation Graph Operating System Principles 7. 32 Silberschatz, Galvin and Gagne © 2005

7. 6. 2 Several Instances of a Resource Type n Available: A vector of length m indicates the number of available resources of each type. n Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. n Request: An n x m matrix indicates the current request of each process. If Request [i][j] = k, then process Pi is requesting k more instances of resource type. Rj. Operating System Principles 7. 33 Silberschatz, Galvin and Gagne © 2005

Detection Algorithm 1. Let Work and Finish be vectors of length m and n. 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 Algorithm requires an order of O(m x n 2) operations to detect whether the system is in deadlocked state. 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. Operating System Principles 7. 34 Silberschatz, Galvin and Gagne © 2005

Example of Detection Algorithm n Five processes: P 0 through P 4; n three resource types A (7 instances), B (2 instances), and C (6 instances). n Snapshot at time T 0: Allocation Request Available ABC ABC P 0 010 000 P 1 200 202 P 2 303 000 P 3 211 100 P 4 002 n Sequence <P 0, P 2, P 3, P 1, P 4> will result in Finish[i] = true for all i. Operating System Principles 7. 35 Silberschatz, Galvin and Gagne © 2005

Example (Cont. ) n P 2 requests an additional instance of type C. Request ABC P 0 000 P 1 201 P 2 001 P 3 100 P 4 002 n State of system? l Can reclaim resources held by process P 0, but insufficient resources to fulfill other processes’ requests. l Deadlock exists, consisting of processes P 1, P 2, P 3, and P 4. Operating System Principles 7. 36 Silberschatz, Galvin and Gagne © 2005

7. 6. 3 Detection-Algorithm Usage n When, and how often, to invoke depends on: l How often a deadlock is likely to occur? l How many processes will need to be rolled back? 4 one for each disjoint cycle n We could invoke the deadlock detection l every time a request cannot be granted immediately l once per hour l whenever the CPU utilization drops below 40 percent n 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. Operating System Principles 7. 37 Silberschatz, Galvin and Gagne © 2005

7. 7 Recovery from Deadlock n What could be done when a deadlock is discovered? l Inform the operator and let the operator handle the deadlock manually. l Let the system recover automatically 4 Abort one or more processes to break the circular wait 4 Preempt some resources from one or more of the deadlocked processes Operating System Principles 7. 38 Silberschatz, Galvin and Gagne © 2005

Process termination n Abort all deadlocked processes n Abort one process at a time until the deadlock cycle is eliminated l Choose the one that will incur the minimum cost n In which order should we choose to abort? l Priority of the process l How long process has computed, and how much longer to completion l How many and type of resources the process has used. l How many more resources the process needs to complete l How many processes will need to be terminated l Is process interactive or batch Operating System Principles 7. 39 Silberschatz, Galvin and Gagne © 2005

Resource Preemption n Selecting a victim – minimize cost. n Rollback – return to some safe state, restart process for that state. n Starvation free – same process may always be picked as victim. l include number of rollbacks in the cost factor. Operating System Principles 7. 40 Silberschatz, Galvin and Gagne © 2005