Chapter 7: Deadlocks. Operating System Concepts 9 th Edition

Chapter 7: Deadlocks Silberschatz, Galvin and Gagne 2013

Chapter Objectives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of different methods for preventing or avoiding deadlocks in a computer system 7.2 Silberschatz, Galvin and Gagne 2013

Chapter 7: Deadlocks System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock 7.3 Silberschatz, Galvin and Gagne 2013

Warm-up Learning from Mistakes A Comprehensive Study on Real World Concurrency Bug Characteristics Shan Lu, Soyeon Park, Eunsoo Seo, Yuanyuan Zhou ASPLOS 08, March 2008, Seattle, Washington Non-Deadlock Bugs A large fraction (97%) of non-deadlock bugs are either atomicity or order violations. 7.4 Silberschatz, Galvin and Gagne 2013

Atomicity violation bugs Warm-up Order-Violation Bugs 7.5 Silberschatz, Galvin and Gagne 2013

System Model System consists of resources Resource types R 1, R 2,..., R m CPU cycles, memory space, I/O devices Each resource type R i has W i instances. Each process utilizes a resource as follows: request use release 7.7 Silberschatz, Galvin and Gagne 2013

Deadlock Characterization Deadlock can arise if four conditions hold simultaneously. 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 additional resources held by other processes No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task 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 1 is waiting for a resource that is held by P n, and P n is waiting for a resource that is held by P 0. 7.9 Silberschatz, Galvin and Gagne 2013

Deadlock with Mutex Locks 7.10 Silberschatz, Galvin and Gagne 2013

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 7.11 Silberschatz, Galvin and Gagne 2013

Resource-Allocation Graph (Cont.) Process Resource Type with 4 instances P i requests instance of R j P i P i is holding an instance of R j R j P i R j 7.12 Silberschatz, Galvin and Gagne 2013

Example of a Resource Allocation Graph Deadlock? 7.13 Silberschatz, Galvin and Gagne 2013

Resource Allocation Graph With A Deadlock Deadlock? 7.14 Silberschatz, Galvin and Gagne 2013

Graph With A Cycle But No Deadlock Deadlock? 7.15 Silberschatz, Galvin and Gagne 2013

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 7.16 Silberschatz, Galvin and Gagne 2013

Why Do Deadlocks Occur? Complex dependencies arise between components in large code bases. The nature of encapsulation. Modularity does not mesh well with locking. 7.17 Silberschatz, Galvin and Gagne 2013

Methods for Handling Deadlocks Ensure that the system will never enter a deadlock state: Deadlock prevention Deadlock avoidence Allow the system to enter a deadlock state and then recover Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX 7.19 Silberschatz, Galvin and Gagne 2013

Deadlock Prevention Circular Wait impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration total ordering partial ordering An ordering, or hierarchy, does not in itself prevent deadlock Enforcing lock ordering by lock address 7.21 Silberschatz, Galvin and Gagne 2013

Deadlock Prevention Hold and Wait must guarantee that whenever a process requests a resource, it does not hold any other resources 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 allocated to it. Low resource utilization; starvation possible Note that the solution is problematic: Encapsulation works against us Decreased concurrency 7.22 Silberschatz, Galvin and Gagne 2013

No Preemption Deadlock Prevention (Cont.) If a process that is holding some resources requests another resource that cannot be immediately allocated to it, 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 Problem: Encapsulation works against us Livelock 7.23 Silberschatz, Galvin and Gagne 2013

Livelock The livelock problem: It is possible (though perhaps unlikely) that two threads could both be repeatedly attempting this sequence and repeatedly failing to acquire both locks. Solution to the livelock problem: add a random delay before looping back and trying the entire thing over again. 7.24 Silberschatz, Galvin and Gagne 2013

Deadlock Prevention Mutual Exclusion not required for sharable resources (e.g., read-only files); must hold for non-sharable resources Wait-free concurrency atomically increment a value by a certain amount 7.25 Silberschatz, Galvin and Gagne 2013

Deadlock Prevention Wait-free concurrency (contd.) list insertion: inserts at the head of a list No lock is acquired, and no deadlock can arise (though livelock is still a possibility). 7.26 Silberschatz, Galvin and Gagne 2013

Deadlock Avoidance Deadlock Avoidance via Scheduling Example: Assume we have two processors and four threads which must be scheduled upon them. Assume further we know that Thread 1 (T1) grabs locks L1 and L2 (in some order, at some point during its execution), T2 grabs L1 and L2 as well, T3 grabs just L2, and T4 grabs no locks at all. What if T3 grabs L1 and L2? 7.28 Silberschatz, Galvin and Gagne 2013

Deadlock Avoidance Requires that the system has some additional a priori information available Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes 7.29 Silberschatz, Galvin and Gagne 2013

Safe State When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state System is in safe state if there exists a sequence <P 1, P 2,, P n > of ALL the processes in the systems 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 resource needs are not immediately available, then P i can wait until all 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, and so on 7.30 Silberschatz, Galvin and Gagne 2013

Basic Facts If a system is in safe state no deadlocks If a system is in unsafe state possibility of deadlock Avoidance ensure that a system will never enter an unsafe state. 7.31 Silberschatz, Galvin and Gagne 2013

Safe, Unsafe, Deadlock State 7.32 Silberschatz, Galvin and Gagne 2013

Safe State Consider a system with 12 magnetic tape drives and three processes: P0, P1, andp2. At time t0, the system is in a safe state. The sequence <P1, P0, P2> satisfies the safety condition. Show by an example that a system can go from a safe state to an unsafe state. Suppose that, at time t1, processp2 requests and is allocated one more tape drive. The system is no longer in a safe state. 7.33 Silberschatz, Galvin and Gagne 2013

Avoidance Algorithms Single instance of a resource type Use a resource-allocation graph Multiple instances of a resource type Use the banker s algorithm 7.34 Silberschatz, Galvin and Gagne 2013

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 7.35 Silberschatz, Galvin and Gagne 2013

Resource-Allocation Graph 7.36 Silberschatz, Galvin and Gagne 2013

Unsafe State In Resource-Allocation Graph 7.37 Silberschatz, Galvin and Gagne 2013

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 7.38 Silberschatz, Galvin and Gagne 2013

Banker s Algorithm Multiple instances Naming 为保证资金的安全, 银行家规定 : 需求有界 : 当一个顾客对资金的最大需求量不超过银行家现有的资金时就可接纳该顾客 ; 顾客可以分期贷款, 但贷款的总数不能超过最大需求量 ; 有限等贷 : 当银行家现有的资金不能满足顾客尚需的贷款数额时, 对顾客的贷款可推迟支付, 但总能使顾客在有限的时间里得到贷款 ; 限时还贷 : 当顾客得到所需的全部资金后, 一定能在有限的时间里归还所有的资金. Edsger Wybe Dijkstra 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. 7.39 Silberschatz, Galvin and Gagne 2013

Data Structures for the Banker s Algorithm Let n = number of processes, and m = number 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] 7.40 Silberschatz, Galvin and Gagne 2013

Safety Algorithm 判断 : 能否找到一个进程序列, 依次全部保证完成 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1,, n- 1 2. Find an i such that both: (a) Finish [i] = false (b) Need 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] == true for all i, then the system is in a safe state 7.41 Silberschatz, Galvin and Gagne 2013

Resource-Request Algorithm for Process P i 首先假设分配, 之后判断 : 能否找到一个进程序列, 依次全部保证完成 Request i = 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 i ; Allocation i = Allocation i + Request i ; Need i = Need i Request i ; If safe the resources are allocated to P i If unsafe P i must wait, and the old resource-allocation state is restored 7.42 Silberschatz, Galvin and Gagne 2013

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 Max Available 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 7.43 Silberschatz, Galvin and Gagne 2013

Example (Cont.) 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 < P 1, P 3, P 4, P 2, P 0 > satisfies safety criteria 7.44 Silberschatz, Galvin and Gagne 2013

Example: P 1 Request (1,0,2) Check that Request Available (that is, (1,0,2) (3,3,2) true Allocation Need Available A B C A B C A 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 2 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 < 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? 7.45 Silberschatz, Galvin and Gagne 2013

死锁 : 习题 1 设系统中有三类资源 A B 和 C, 又设系统中有 5 个进程 P1,P2,P3,P4 和 P5 A 资源的数量为 17,B 资源的数量为 5,C 资源的数量为 20 在 T0 时刻系统状态如下 : MAX ALLOCATION AVAILABLE A B C A B C A B C P1 5 5 9 2 1 2 2 3 3 P2 5 3 6 4 0 2 P3 4 0 11 4 0 5 P4 4 2 5 2 0 4 P5 4 2 4 3 1 4 根据银行家算法分析完成以下问题 : (1) T0 时刻系统是否处于安全状态? 如是, 则给出进程安全序列 (2) T0 时刻如果进程 P2 申请 0 个资源类 A 3 个资源类 B 和 4 个资源类 C, 能否实施分配? 为什么? (3) 在 (2) 的基础上如果进程 P4 申请 2 个资源类 A 0 个资源类 B 和 1 个资源类 C, 能否实施分配? 为什么? (4) 在 (3) 的基础上如果进程 P1 申请 0 个资源类 A 2 个资源类 B 和 0 个资源类 C, 能否实施分配? 为什么? 7.46 Silberschatz, Galvin and Gagne 2013

Deadlock Detection Allow system to enter deadlock state Detection algorithm Recovery scheme 7.48 Silberschatz, Galvin and Gagne 2013

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 7.49 Silberschatz, Galvin and Gagne 2013

Resource-Allocation Graph and Wait-for Graph Resource-Allocation Graph Corresponding wait-for graph 7.50 Silberschatz, Galvin and Gagne 2013

Several 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. 7.51 Silberschatz, Galvin and Gagne 2013

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 7.52 Silberschatz, Galvin and Gagne 2013

Detection Algorithm (Cont.) 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 Algorithm requires an order of O(m x n 2 ) operations to detect whether the system is in deadlocked state 7.53 Silberschatz, Galvin and Gagne 2013

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 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 <P 0, P 2, P 3, P 1, P 4 > will result in Finish[i] = true for all i 7.54 Silberschatz, Galvin and Gagne 2013

Example (Cont.) P 2 requests an additional instance of type C State of system? Request A B C P 0 0 0 0 P 1 2 0 2 P 2 0 0 1 P 3 1 0 0 P 4 0 0 2 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 So we say P 2 causes the deadlock. 7.55 Silberschatz, Galvin and Gagne 2013

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. 7.56 Silberschatz, Galvin and Gagne 2013

Recovery from Deadlock: Process Termination Abort all deadlocked processes Abort one process at a time until the deadlock cycle is eliminated In which order should we choose to abort? 1. Priority of the process 2. How long process has computed, and how much longer to completion 3. Resources the process has used 4. Resources process needs to complete 5. How many processes will need to be terminated 6. Is process interactive or batch? 7.58 Silberschatz, Galvin and Gagne 2013

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 7.59 Silberschatz, Galvin and Gagne 2013

Review Describe the four necessary conditions of a deadlock. Tell the relations and differences between preventing, avoiding, detecting, and recovering from a deadlock. 实验 : 银行家算法的实现 1. 在 Windows 操作系统上, 利用 Win32 API 编写多线程应用程序实现银行家算法 2. 创建 n 个线程来申请或释放资源, 只有保证系统安全, 才会批准资源申请 3. 通过 Win32 API 提供的信号量机制, 实现共享数据的并发访问 7.60 Silberschatz, Galvin and Gagne 2013

End of Chapter 7 Silberschatz, Galvin and Gagne 2013