Conditions for Deadlock in Operating System

Deadlock in an operating system is a condition where a group of processes become stuck in a state where no process in the group can proceed.

This happens because each process is waiting for a resource that another process in the group holds, creating a cycle of waiting where no process can continue its execution.

Essentially, the processes are deadlocked because they cannot release the resources they need to continue.

Conditions for Deadlock in Operating System

In an operating system, deadlock occurs when a set of processes are blocked, each waiting for a resource held by another process in the set, leading to a situation where no process can proceed.

For a deadlock to happen, certain conditions must be met. These conditions are commonly referred to as the deadlock conditions and are based on the Coffman conditions, which describe the necessary circumstances for a deadlock to occur.

• Mutual Exclusion: At least one resource must be held in a non-shareable mode, meaning that only one process can use a resource at any given time. If another process requests the same resource, it must wait for the resource to be released.

  Example: A printer is a non-shareable resourceonly one process can print at a time.

• Hold and Wait: A process holding at least one resource is waiting to acquire additional resources that are currently being held by other processes. This means that a process that already has resources may request more resources, which it cannot obtain because they are held by other processes. Example: Process A holds a printer and waits for a scanner, while process B holds a scanner and waits for the printer.

• No Preemption: Resources cannot be forcibly taken away from a process holding them. A process can only release a resource voluntarily. If a process is holding a resource and is waiting for another, it cannot be preempted (i.e., it cannot be forced to release the resource until it finishes its task).

  Example: If Process A has a resource (e.g., a file) and is waiting for a printer, the operating system cannot forcibly take the file from Process A to assign it to another process.

• Circular Wait: A set of processes are waiting for each other in a circular chain. In other words, each process in the set is waiting for a resource held by the next process in the chain.

  Example: Process A waits for a resource held by Process B, Process B waits for a resource held by Process C, and Process C waits for a resource held by Process A, forming a cycle of waiting processes.

How to avoid deadlock?

To avoid deadlock, the operating system can take measures to prevent one or more of the four necessary conditions from occurring. This can be done in the following ways −

• Prevent Mutual Exclusion: This is generally not feasible because many resources (like printers, files, or memory) must be used exclusively by one process at a time. However, for some resources (like read-only resources), multiple processes can access them simultaneously.

• Prevent Hold and Wait: Require processes to request all resources they will need at once before starting execution. This is called the "all-or-nothing" approach. Another option is to require processes to release all the resources they currently hold before requesting new ones.

• Prevent No Preemption: Allow preemption of resources. If a process holding some resources is waiting for additional resources, the operating system can preempt the resources currently held by the process and assign them to other processes.