Deadlock in operating system

In computing, a 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. Deadlocks can occur in any type of system that uses multiple processes or threads, but they are particularly common in operating systems.

A deadlock occurs when two or more processes are each waiting for the other to release a resource that they need in order to proceed. For example, imagine that Process A holds Resource X and is waiting for Resource Y, while Process B holds Resource Y and is waiting for Resource X. Neither process can proceed until it has access to both resources, so they are stuck in a deadlock.

Deadlocks can be difficult to detect and resolve because they occur when a system is in a specific state, and that state may be difficult to reproduce or identify. In order to prevent or resolve deadlocks, operating systems and other software systems use a variety of techniques and algorithms.

In this article, we will explore what deadlocks are, how they occur, and the different techniques used to prevent and resolve them.

Causes of Deadlocks

Deadlocks occur when two or more processes are each waiting for the other to release a resource that they need in order to proceed. There are four conditions that must be present in order for a deadlock to occur:

1. Mutual exclusion: At least one resource must be held in a non-shareable mode. This means that only one process at a time can use the resource.

2. Hold and wait: A process must be holding at least one resource and waiting to acquire additional resources.

3. No preemption: Resources cannot be forcibly taken away from a process.

4. Circular wait: A circular chain of two or more processes exists, where each process is holding a resource that the next process in the chain needs.

If these four conditions are met, a deadlock can occur. Let's take a closer look at each of these conditions.

Mutual Exclusion

In order for a deadlock to occur, at least one resource must be held in a non-shareable mode. This means that only one process at a time can use the resource. Examples of resources that are typically held in a non-shareable mode include printers, disk drives, and other hardware devices.

Hold and Wait

The hold and wait condition occurs when a process is holding at least one resource and waiting to acquire additional resources. For example, imagine that Process A is holding Resource X and wants to acquire Resource Y, while Process B is holding Resource Y and wants to acquire Resource X. If both processes wait indefinitely for the other to release its resource, a deadlock occurs.

No Preemption

The no preemption condition means that resources cannot be forcibly taken away from a process. This means that if a process is holding a resource and another process requests that resource, the first process cannot be preempted and forced to release the resource.

Circular Wait

The circular wait condition occurs when there is a circular chain of two or more processes, where each process is holding a resource that the next process in the chain needs. For example, imagine that Process A is holding Resource X and waiting for Resource Y, while Process B is holding Resource Y and waiting for Resource Z, and Process C is holding Resource Z and waiting for Resource X. If all three processes wait indefinitely for the resource that the next process in the chain is holding, a deadlock occurs.

Deadlock Prevention

There are several techniques that operating systems and other software systems can use to prevent deadlocks from occurring. These techniques include:

1. Deadlock Avoidance

Deadlock avoidance involves carefully monitoring the state of the system and predicting when deadlocks may occur. This allows the system to take action before a deadlock occurs. For example, the system may refuse to grant a request for a resource if it would result in a deadlock. Deadlock avoidance requires that the system have a good understanding of the resource requirements of each process and the current state of the system.

2. Resource Allocation Graph

A resource allocation graph is a way of representing the resource allocation and request relationships between processes. Each process is represented by a circle, and each resource is represented by a square. Arrows between the circles and squares represent requests and allocations. A cycle in the graph indicates the potential for a deadlock. Operating systems can use resource allocation graphs to detect and prevent deadlocks.

3. Banker's Algorithm

The Banker's algorithm is a deadlock avoidance algorithm that is used in operating systems to allocate resources to processes. The algorithm works by simulating the allocation of resources and checking if this would result in a deadlock. If a request would result in a deadlock, the request is denied.

4. Timeouts

Timeouts are a simple but effective way of preventing deadlocks. A process that is waiting for a resource can be given a time limit to wait. If the resource is not available within the time limit, the process can be aborted, and the resources it was holding can be released. This prevents deadlocks by ensuring that processes that are waiting for resources do not wait indefinitely.

Deadlock Detection

In addition to preventing deadlocks from occurring, operating systems can also detect when a deadlock has occurred. Deadlock detection involves examining the state of the system to see if any deadlocks exist. There are several techniques that can be used to detect deadlocks, including:

1. Resource Allocation Graph

As mentioned earlier, a resource allocation graph can be used to detect deadlocks. If a cycle exists in the graph, a deadlock has occurred.

2. Deadlock Detection Algorithm

A deadlock detection algorithm is a software algorithm that is used to detect deadlocks. The algorithm works by examining the resource allocation state of the system to determine if a deadlock has occurred. If a deadlock is detected, the algorithm can take action to resolve the deadlock.

3. Timeout

Timeouts can also be used to detect deadlocks. If a process is waiting for a resource for too long, it may indicate that a deadlock has occurred.

Deadlock Resolution

Once a deadlock has been detected, the operating system must take action to resolve the deadlock. There are several techniques that can be used to resolve deadlocks, including:

1. Process Termination

Process termination involves terminating one or more processes that are involved in the deadlock. By terminating a process, the resources that it was holding are released, which can break the deadlock.

2. Resource Preemption

Resource preemption involves forcibly taking a resource away from a process. By doing so, the resource can be given to another process that needs it, which can break the deadlock.

3. Rollback

Rollback involves undoing the work that was done by one or more processes in order to release the resources that they were holding. This can be a complex and time-consuming process, but it can be effective in resolving deadlocks.

Conclusion

Deadlocks can be a serious problem in operating systems and other software systems. They occur when two or more processes are each waiting for the other to release a resource that they need in order to proceed. Deadlocks can be difficult to detect and resolve, but there are several techniques that can be used to prevent, detect, and resolve deadlocks. These techniques include deadlock avoidance, resource allocation graphs, the Banker's algorithm, timeouts, deadlock detection algorithms, process termination, resource preemption, and rollback. By using these techniques, operating systems can ensure that deadlocks do not occur or are quickly resolved if they do occur.