Conditions for Deadlock in Operating System

Condition for Deadlock

In the system, the deadlock occurs when the system has these four conditions simultaneously that are as follows:

• Mutual Exclusion
• No-preemption
• Hold & wait
• Circular wait

For a deadlock to occur, all four conditions must be met. The impasse is broken if any one of them is averted or resolved.

Mutual Exclusion

When two people meet on the landings, they cannot just stroll through since there is only enough room for one person. The first prerequisite for the occurrence of the deadlock is to enable only one person (or process) to utilize the step between them (or the resource).

• The resource has to be allocated to only one process, which is freely available.
• There should be a one-to-one relationship between the resource and process.

No-preemption

To overcome the deadlock, just cancel one of the processes and let the others continue. However, the Operating System does not. It provides resources to the processors for as long as the job requires till it is accomplished. As a result, there is no need for a temporary reallocation of resources. It's the third deadlock condition.

• The resource has to be voluntarily released by the process after completing the execution.
• It is not allowed to preempt the resource from the process forcefully.

Hold & wait

Holding occurs when two individuals refuse to retreat and defend their territory. This is the next condition that must be met in order to break the deadlock.

• A process acquires resources piecemeal. While it awaits allocation and assignment of other resources, it holds on to its acquired resources.
• In other words, a process waits for some resources while holding another resource simultaneously.

Circular wait

Circular wait occurs when two persons refuse to retreat and wait for each other to recede so that they can finish their work. It's the final thing that can cause a deadlock. Processes waiting for resources from the others form a circular chain. In that chain, each process holds at least one unit resource, which is demanded by the next process in the chain.

Suppose there exists a set {P₀, P₁,..., P₀} of waiting processes such that the process P₀ is waiting for a resource that P1 holds, and the process P₁ is waiting for a resource that P2 holds. The resource allocation graph is:

Symbol and representation

The circle will represent the process as 

Resources will be represented with rectangle blocks as

If the process P₁ is requested a resource, then it is represented as

If the process P₁ is holding an instance, then it is represented as

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