What is a Deadlock?

How to avoid Deadlocks

Deadlocks can be avoided by avoiding at least one of the four conditions, because all this four conditions are required simultaneously to cause deadlock.

1. Mutual Exclusion
   Resources shared such as read-only files do not lead to deadlocks but resources, such as printers and tape drives, requires exclusive access by a single process.
2. Hold and Wait
   In this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others.
3. No Preemption
   Preemption of process resource allocations can avoid the condition of deadlocks, where ever possible.
4. Circular Wait
   Circular wait can be avoided if we number all resources, and require that processes request resources only in strictly increasing(or decreasing) order.

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Handling Deadlock

The above points focus on preventing deadlocks. But what to do once a deadlock has occured. Following three strategies can be used to remove deadlock after its occurrence.

1. Preemption
   We can take a resource from one process and give it to other. This will resolve the deadlock situation, but sometimes it does causes problems.
2. Rollback
   In situations where deadlock is a real possibility, the system can periodically make a record of the state of each process and when deadlock occurs, roll everything back to the last checkpoint, and restart, but allocating resources differently so that deadlock does not occur.
3. Kill one or more processes
   This is the simplest way, but it works.

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What is a Livelock?

There is a variant of deadlock called livelock. This is a situation in which two or more processes continuously change their state in response to changes in the other process(es) without doing any useful work. This is similar to deadlock in that no progress is made but differs in that neither process is blocked or waiting for anything.

A human example of livelock would be two people who meet face-to-face in a corridor and each moves aside to let the other pass, but they end up swaying from side to side without making any progress because they always move the same way at the same time.