Deadlock

A deadlock occurs when two or more processes are in a waiting state, each holding at least one resource and waiting for other resources that are being held by another process. This creates a cycle of dependency, where no process can proceed because they are each waiting for resources that are locked by others.

Conditions for Deadlock

For a deadlock to occur, four necessary conditions must hold simultaneously, often referred to as Coffman’s Conditions:

1. Mutual Exclusion: At least one resource must be non-shareable, meaning only one process can use it at any given time.
2. Hold and Wait: A process holding one or more resources can request additional resources and wait while holding the already allocated ones.
3. No Preemption: Resources cannot be forcibly taken from processes. They can only be released voluntarily by the process holding them.
4. Circular Wait: A circular chain of processes exists where each process is waiting for a resource held by the next process in the chain.

Deadlock Prevention and Avoidance

There are several strategies to prevent or avoid deadlock in concurrent systems:

1. Deadlock Prevention: This strategy aims to ensure that at least one of the four conditions for deadlock cannot hold:

• Mutual Exclusion: Some resources are inherently non-shareable, but for others, allowing concurrent access can prevent deadlocks.
• Hold and Wait: Processes must request all necessary resources at once, preventing them from holding some resources while waiting for others.
• No Preemption: If a process holding some resources is denied a further resource request, it must release its current resources.
• Circular Wait: Impose an ordering on resources, and processes can only request resources in a predefined order, breaking the circular chain.

1. Deadlock Avoidance: This involves careful resource allocation based on a system’s state, ensuring that resource requests do not lead to deadlock. The Banker’s Algorithm, developed by Edsger Dijkstra, is a well-known deadlock avoidance strategy that allocates resources dynamically while ensuring that the system remains in a safe state.

Deadlock Detection and Recovery

In systems where deadlock prevention and avoidance are impractical, deadlock detection can be used. This approach allows deadlock to occur but regularly checks for its presence by analyzing resource allocation graphs for cycles.

Once a deadlock is detected, recovery techniques can be used, including:

• Process Termination: Terminating one or more processes involved in the deadlock to break the cycle.
• Resource Preemption: Forcibly taking resources from some processes and reallocating them to others.