How can deadlock be resolved

Deadlock can occur when the permanent blocking of a set of processes compete for the same system resources. A set of processes is deadlocked when each process in the set is blocked awaiting an event that can only be triggered by another blocked process in the set.

Deadlock is permanent because none of the events are ever triggered. Three conditions must take place for a deadlock to take place. The first one is Mutual exclusion, which is a single process that uses one resource at a time. No process may access a resource unit that is being utilized by another process.

Hold and wait is the second condition, it can be described as one process that holds assigned resources while waiting for another assignment. Finally, No preemption occurs when no resource can be forced or removed from a process holding it. These three conditions are necessary for a deadlock to exist. However, a fourth condition is required for an actual deadlock to take place. Circular wait occurs when a closed chain of processes exists, and each process holds one or more resources needed by the next process in the chain.

This condition is an immediate result of the first three. Below is a self-explanatory illustration of Deadlock. Deadlock blocks a set of processes that competes for system resources. This can be permanent unless the OS takes action, such as forcing one or more processes to backtrack. Deadlock may involve consumable or reusable resources. A reusable resource is one that is not depleted by use.

A consumable resource is one that is destroyed when it is obtained by a process. There are three approaches to dealing with deadlock: prevention, detection, and avoidance. Prevention guarantees that deadlocks will not happen. Detection is required if the OS is willing to grant resource requests; the OS checks for deadlocks and takes action to break the deadlock.

Avoidance involves the analysis of each new resource request to determine if it could lead to deadlock, and granting it only if deadlock is not an option. For Windows you can run driver verifier to scan for any corrupted drivers, which may be causing problems, this program works by running various stress tests on drivers, in order to produce a BSOD, which will locate the driver.

To the best of my knowledge deadlocks are ignored by Linux operating systems. Under the deadlock detection, deadlocks are allowed to occur. Then the state of the system is examined to detect that a deadlock has occurred and subsequently it is corrected. An algorithm is employed that tracks resource allocation and process states, it rolls back and restarts one or more of the processes in order to remove the detected deadlock. By Removing the mutual exclusion condition means that no process will have exclusive access to a resource.

Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process. A thread is a flow of execution through the process code, with its own program counter that keeps track of which instruction to execute next, system registers which hold its current working variables, and a stack which contains the execution history. Each thread represents a separate flow of control.

Election algorithm and distributed processing. Distributed Algorithm is a algorithm that runs on a distributed system. Election algorithms are designed to choose a coordinator. Election Algorithms : Election algorithms choose a process from group of processors to act as a coordinator. Two types of deadlocks can be considered: 1. Resource Deadlock.

Occurs when processes are trying to get exclusive access to devices, files, locks, servers, or other resources. In Resource deadlock model, a process waits until it has received all the resources that it has requested. Four Necessary and Sufficient Conditions for Deadlock mutual exclusion.

The resources involved must be unshareable; otherwise, the processes would not be prevented from using the resource when necessary. A deadlock happens when two or more transactions block each other by holding locks on resources that each of the transactions also need.

For example: Transaction 1 holds a lock on Table A. The OS can detect the deadlocks with the help of Resource allocation graph. In single instanced resource types, if a cycle is being formed in the system then there will definitely be a deadlock.

On the other hand, in multiple instanced resource type graph, detecting a cycle is not just enough. It chooses a victim from the transactions who are involved in the deadlock, forces it to rollback all actions. As can be seen from this explanation, deadlock in SQL Server is a special contention problem; also, each deadlock has a unique characteristic, so the solution has different approaches according to problem characteristics.

In this real scenario, an in-house application returns an error to the users, and users notify the development team about this error. The development team realizes that it is a deadlock issue, but they could not find the main reason for the problem. Under these circumstances, the team decides to receive consultancy service from an experienced database administrator. In the next sections, we will learn how database administrators analyze and resolve this deadlock problem.

The development team was using the following table to store the order numbers, and the following query was used to create the first row of the day. The data structure of the table below is similar to the below illustration; namely, there is only one row per day.

The database administrator decided to research the deadlocks problems because of the error message. The error message obviously was indicating a deadlock problem. Transaction Process ID XX was deadlocked on lock resources with another process and has been chosen as the deadlock victim. Rerun the transaction. Prevention Deadlock prevention works by preventing one of the four Coffman conditions from occurring.

Removing the mutual exclusion condition means that no process will have exclusive access to a resource. This proves impossible for resources that cannot be spooled. But even with spooled resources, the deadlock could still occur. Algorithms that avoid mutual exclusion are called non-blocking synchronization algorithms. The hold and wait or resource holding conditions may be removed by requiring processes to request all the resources they will need before starting up or before embarking upon a particular set of operations.

This advance knowledge is frequently difficult to satisfy and, in any case, is an inefficient use of resources. Another way is to require processes to request resources only when it has none; First they must release all their currently held resources before requesting all the resources they will need from scratch. This too is often impractical.

It is so because resources may be allocated and remain unused for long periods. Also, a process requiring a popular resource may have to wait indefinitely, as such a resource may always be allocated to some process, resulting in resource starvation. The no preemption condition may also be difficult or impossible to avoid as a process has to be able to have a resource for a certain amount of time, or the processing outcome may be inconsistent or thrashing may occur.

However, the inability to enforce preemption may interfere with a priority algorithm.