Computer Science
Deadlock
Deadlock is a situation in which two or more processes are unable to proceed because each is waiting for one of the others to do something. This can occur when two or more processes are competing for the same resources or when a process is unable to release a resource that it has already acquired. Deadlocks can cause a system to become unresponsive and can be difficult to resolve.
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9 Key excerpts on "Deadlock"
- MengChu Zhou, Maria Pia Fanti(Authors)
- 2018(Publication Date)
- CRC Press(Publisher)
1 Introduction to Deadlock Research in Computer-integrated Systems Maria Pia Fanti Department of Elettrotecnica ed Elettronica, Politecnico di Bari, Bari, Italy. MengChu Zhou Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, NJ, USA 1.1 INTRODUCTION Computer-integrated systems are characterized by a structure in which a set of executing processes share (or compete for) a set of resources, for example, processes running in a multitask operating system, transactions in distributed data base systems, concurrent software systems and computer networks, parts produced by automated manufacturing systems, and vehicles competing for the same path in transportation systems. The interacting parts and shared resources are character ized as a resource allocation system that can lead to problems that the isolated execution of a task does not have. Among these problems, we consider Deadlock situations: Deadlock is a phenomenon in which a system or a part of it remains indefinitely blocked and a set of jobs cannot terminate its task. To demonstrate this condition, we consider a simple and intuitive example of traffic flow along the edges of a square, depicted in Figure 1.1. The boxes represent the cars and the arrows the directions in which they have to move. In this situation, each car of the displayed set is blocked by another car in the same set, occupying the space that it requires. Hence, this situation represents a Deadlock. 1 2 M. P. Fanti and M. C. Zhou Figure 1.1: Deadlock condition in traffic flow Deadlock is a logical condition arising in Discrete Event Dynamic Systems (DEDS) [4] and it is caused by an inappropriate allocation of resources to concur rent executing processes. Deadlock is first addressed by computer scientists devel oping resource allocation logic in operating systems [7,21,23,25,29].
eBook - ePubReal-Time Embedded Systems
Open-Source Operating Systems Perspective
- Ivan Cibrario Bertolotti, Gabriele Manduchi(Authors)
- 2017(Publication Date)
- CRC Press(Publisher)
B or vice versa.Unfortunately, this means that the code will be hard to debug, and even the insertion of a debugger or code instrumentation to better understand what is happening may perturb system timings enough to make the Deadlock disappear. This is a compelling reason to address Deadlock problems from a theoretical perspective, during system design , rather than while testing or debugging it.It also depends on a few specific properties of the resources involved and on how the operating system manages them. For example, albeit this technique is not widespread in real-time operating systems, some general-purpose operating systems are indeed able to swap process images in and out of main memory with the assistance of a mass storage device. Doing this, they are able to accommodate processes whose total memory requirements exceed the available memory.In this case, the memory request performed by B in our running example does not necessarily lead to an endless wait because the operating system can temporarily take away—or preempt —some memory from A in order to satisfy B ’s request, so that both process will be eventually able to complete their execution. As a consequence, the same processes may or may not be at risk for what concerns Deadlock, when they are executed by operating systems employing dissimilar memory management or, more generally, resource management techniques.4.2 Formal Definition of Deadlock
In the most general way, a Deadlock can be defined formally as a situation in which a set of processes passively waits for an event that can be triggered only by another process in the same set. More specifically, when dealing with resources, there is a Deadlock when all processes in a set are waiting for some resources previously allocated to other processes in the same set. As discussed in the example of Section 4.1
eBook - ePub- Jie Wu(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
Chapter 5Prevention, Avoidance, and Detection of Deadlock
Distributed systems, in general, exhibit a high degree of resource and data sharing, a situation in which Deadlocks may happen. Deadlocks arise when members of a group of processes which hold resources are blocked indefinitely from access to resources held by other processes within the group. In this chapter methods for prevention, avoidance, and detection of Deadlock in distributed systems are discussed.5.1 The Deadlock Problem
In a computer system Deadlocks arise when members of a group of processes which hold resources are blocked indefinitely from access to resources held by other processes within the group. For example, consider a system with two I/O controllers. The system will immediately allocate an I/O controller to a requesting process upon each request. Suppose two processes P i and P j have the following requests:- P i requests one I/O controller and the system allocates one.
- P j requests one I/O controller and again the system allocates one.
- P i wants another I/O controller but has to wait since the system ran out of I/O controllers.
- P j wants another I/O controller and waits.
In the above example both processes are in the state of permanent blocking and a Deadlock involving these two processes occurs.FIGURE 5.1 Two cities connected by (a) one bridge and by (b) two bridges.5.1.1 Conditions for Deadlocks
Formally, a Deadlock can arise if and only if the following four conditions hold simultaneously [14 ]:- Mutual exclusion . No resource can be shared by more than one process at a time.
- Hold and wait . There must exist a process that is holding at least one resource and is waiting to acquire additional resources that are currently being held by other processes.
- No preemption . A resource cannot be preempted.
- Circular wait . There is a cycle in the wait-for graph.
Figure 5.1 shows a Deadlock example where there are two cities A and B separated by a river. The two cities are connected by a bridge (Figure 5.1 (a) ). Assume that the bridge is so narrow that only one person can walk through at a time. There will be no problem if several persons walk towards the same direction. Deadlock occurs when two persons heading in different directions - one from city A to city B and the other one from city B to city A -
eBook - PDFDistributed Computing
Principles, Algorithms, and Systems
- Ajay D. Kshemkalyani, Mukesh Singhal(Authors)
- 2011(Publication Date)
- Cambridge University Press(Publisher)
C H A P T E R 10 Deadlock detection in distributed systems 10.1 Introduction Deadlocks are a fundamental problem in distributed systems and Deadlock detection in distributed systems has received considerable attention in the past. In distributed systems, a process may request resources in any order, which may not be known a priori, and a process can request a resource while holding others. If the allocation sequence of process resources is not controlled in such environments, Deadlocks can occur. A Deadlock can be defined as a condition where a set of processes request resources that are held by other processes in the set. Deadlocks can be dealt with using any one of the following three strategies: Deadlock prevention, Deadlock avoidance, and Deadlock detection. Deadlock prevention is commonly achieved by either having a process acquire all the needed resources simultaneously before it begins execution or by pre-empting a process that holds the needed resource. In the Deadlock avoidance approach to distributed systems, a resource is granted to a process if the resulting global system is safe. Deadlock detection requires an examination of the status of the process–resources interaction for the presence of a Deadlock condition. To resolve the Deadlock, we have to abort a Deadlocked process. In this chapter, we study several distributed Deadlock detection techniques based on various strategies. 10.2 System model A distributed system consists of a set of processors that are connected by a communication network. The communication delay is finite but unpredictable. A distributed program is composed of a set of n asynchronous processes P 1 , P 2 , , P i , , P n that communicate by message passing over the commu-nication network. Without loss of generality we assume that each process is running on a different processor. The processors do not share a common 352 353 10.3 Preliminaries global memory and communicate solely by passing messages over the com-munication network.
eBook - PDF- Ann McHoes, , Ida M. Flynn, , Ann McHoes, Ida M. Flynn(Authors)
- 2017(Publication Date)
- Cengage Learning EMEA(Publisher)
Key Terms avoidance: the dynamic strategy of Deadlock avoidance that attempts to ensure that resources are never allocated in such a way as to place a system in an unsafe state. circular wait: one of four conditions for Deadlock through which each process involved is waiting for a resource being held by another; each process is blocked and can’t con-tinue, resulting in Deadlock. Deadlock: a problem occurring when the resources needed by some jobs to finish execution are held by other jobs, which, in turn, are waiting for other resources to become available. dedicated devices: a system resource that can be assigned to only one job at a time, and then serving that job for the entire time that the job is active. It cannot be shared by multiple jobs at the same time. Copyright 2018 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. WCN 02-300 165 Key Terms detection: the process of examining the state of an operating system in order to deter-mine whether a Deadlock exists. directed graphs: a graphic model representing various states of resource allocations. livelock: a locked system whereby two or more processes continually block the forward progress of the others without making any forward progress themselves. It is similar to a Deadlock except that neither process is blocked or obviously waiting; both are in a continuous state of change. locking: a technique used to guarantee the integrity of the data in a database, through which the user locks out all other users while working with the database. mutual exclusion: one of four conditions for Deadlock in which only one process is allowed to have access to a resource. no preemption: one of four conditions for Deadlock in which a process is allowed to hold on to resources while it is waiting for other resources to finish execution.
eBook - PDF- Abraham Silberschatz, Peter B. Galvin, Greg Gagne(Authors)
- 2020(Publication Date)
- Wiley(Publisher)
• We can use a protocol to prevent or avoid Deadlocks, ensuring that the system will never enter a Deadlocked state. • We can allow the system to enter a Deadlocked state, detect it, and recover. The first solution is the one used by most operating systems, including Linux and Windows. It is then up to kernel and application developers to write programs that handle Deadlocks, typically using approaches outlined in the second solution. Some systems — such as databases — adopt the third solution, allowing Deadlocks to occur and then managing the recovery. Next, we elaborate briefly on the three methods for handling Deadlocks. Then, in Section 8.5 through Section 8.8, we present detailed algorithms. Before proceeding, we should mention that some researchers have argued that none of the basic approaches alone is appropriate for the entire spectrum of resource- allocation problems in operating systems. The basic approaches can be com- bined, however, allowing us to select an optimal approach for each class of resources in a system. To ensure that Deadlocks never occur, the system can use either a Deadlock- prevention or a Deadlock-avoidance scheme. Deadlock prevention provides a set of methods to ensure that at least one of the necessary conditions (Section 8.3.1) cannot hold. These methods prevent Deadlocks by constraining how requests for resources can be made. We discuss these methods in Section 8.5. Deadlock avoidance requires that the operating system be given addi- tional information in advance concerning which resources a thread will request and use during its lifetime. With this additional knowledge, the operating sys- tem can decide for each request whether or not the thread should wait. To decide whether the current request can be satisfied or must be delayed, the system must consider the resources currently available, the resources currently allocated to each thread, and the future requests and releases of each thread.
- Gedare Bloom, Joel Sherrill, Tingting Hu, Ivan Cibrario Bertolotti(Authors)
- 2020(Publication Date)
- CRC Press(Publisher)
collectively sufficient for a Deadlock to occur. These conditions are of both theoretical and practical significance. From the theoretical point of view, they define Deadlock in a general way, which abstracts away as much as possible from any irrelevant characteristics of the tasks and resources involved. Practically speaking, they have been used as the basis for a family of Deadlock prevention algorithms. These algorithms are all based on the fact that, if an appropriate policy is able to prevent one of the four condition from ever being fulfilled in a system, then no Deadlock can possibly occur in the system by definition. The four conditions are:- Mutual exclusion: Each resource can be used by at most one task at a time, so at any given time a resource can only be either free or assigned to one task. If a task tries to acquire a resource currently assigned to another task, it must wait.
- Hold and Wait: The tasks involved in a Deadlock must have successfully acquired at least one resource in the past, and have not released it yet as they wait for more resources. In other words, they must both hold some resources and wait for others.
- Non-preemption: Any resource involved in a Deadlock cannot be taken away from the task it has been assigned to, unless the task voluntarily releases it.
- Circular wait: The task and resources involved in a Deadlock can be arranged to form a circular chain of wait operations. That is, tasks can be ordered so that the first task is waiting for a resource assigned to the second, the second task is waiting for a resource assigned to the third, and so on up to the last task, which is waiting for a resource assigned to the first.
Another very useful tool for reasoning about Deadlock is the resource allocation graph proposed by Holt [61
eBook - PDF- Abraham Silberschatz, Peter B. Galvin, Greg Gagne(Authors)
- 2014(Publication Date)
- Wiley(Publisher)
Since, in general, it is difficult to determine what a safe state is, the simplest solution is a total rollback: abort the process and then restart it. Although it is more effective to roll back the process only as far as necessary to break the Deadlock, this method requires the system to keep more information about the state of all running processes. 3. Starvation. How do we ensure that starvation will not occur? That is, how can we guarantee that resources will not always be preempted from the same process? In a system where victim selection is based primarily on cost factors, it may happen that the same process is always picked as a victim. As a result, this process never completes its designated task, a starvation situation any practical system must address. Clearly, we must ensure that a process can be picked as a victim only a (small) finite number of times. The most common solution is to include the number of rollbacks in the cost factor. Exercises 335 7.8 Summary A Deadlocked state occurs when two or more processes are waiting indefinitely for an event that can be caused only by one of the waiting processes. There are three principal methods for dealing with Deadlocks: • Use some protocol to prevent or avoid Deadlocks, ensuring that the system will never enter a Deadlocked state. • Allow the system to enter a Deadlocked state, detect it, and then recover. • Ignore the problem altogether and pretend that Deadlocks never occur in the system. The third solution is the one used by most operating systems, including Linux and Windows. A Deadlock can occur only if four necessary conditions hold simultaneously in the system: mutual exclusion, hold and wait, no preemption, and circular wait. To prevent Deadlocks, we can ensure that at least one of the necessary conditions never holds. A method for avoiding Deadlocks, rather than preventing them, requires that the operating system have a priori information about how each process will utilize system resources.
eBook - PDF- Abraham Silberschatz, Peter B. Galvin, Greg Gagne(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
Observe that thread T 4 may release its instance of resource type R 2 . That resource can then be allocated to T 3 , breaking the cycle. In summary, if a resource-allocation graph does not have a cycle, then the system is not in a Deadlocked state. If there is a cycle, then the system may or may not be in a Deadlocked state. This observation is important when we deal with the Deadlock problem. 8.4 Methods for Handling Deadlocks Generally speaking, we can deal with the Deadlock problem in one of three ways: • We can ignore the problem altogether and pretend that Deadlocks never occur in the system. • We can use a protocol to prevent or avoid Deadlocks, ensuring that the system will never enter a Deadlocked state. • We can allow the system to enter a Deadlocked state, detect it, and recover. The first solution is the one used by most operating systems, including Linux and Windows. It is then up to kernel and application developers to write programs that handle Deadlocks, typically using approaches outlined in the second solution. Some systems — such as databases — adopt the third solution, allowing Deadlocks to occur and then managing the recovery. Next, we elaborate briefly on the three methods for handling Deadlocks. Then, in Section 8.5 through Section 8.8, we present detailed algorithms. Before proceeding, we should mention that some researchers have argued that none of the basic approaches alone is appropriate for the entire spectrum of resource- allocation problems in operating systems. The basic approaches can be com- bined, however, allowing us to select an optimal approach for each class of resources in a system. To ensure that Deadlocks never occur, the system can use either a Deadlock- prevention or a Deadlock-avoidance scheme. Deadlock prevention provides a set of methods to ensure that at least one of the necessary conditions (Section 8.3.1) cannot hold. These methods prevent Deadlocks by constraining how requests for resources can be made.
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