Deadlock

Process deadlock

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Deadlock at a four-way-stop

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Two processes, two resources

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Avoiding deadlock

Deadlock is a situation in computer science and operating systems where two or more processes or threads are blocked forever, waiting for each other to release a resource. This condition is significant in the field of concurrency control in database systems, distributed computing, and multi-threading environments. Understanding deadlocks is crucial for designing systems that require high reliability and availability.

Causes

The primary cause of a deadlock is the simultaneous holding and requesting of resources by two or more competing actions. The four necessary conditions for a deadlock, as defined by Edgar F. Codd in 1972, are:

1. Mutual Exclusion: At least one resource must be held in a non-shareable mode. If another process requests this resource, the requesting process must be delayed until the resource is released.
2. Hold and Wait: A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes.
3. No Preemption: Resources cannot be forcibly removed from the processes holding them until the resources are used to completion.
4. Circular Wait: A set of processes must be waiting for each other in a circular chain, where each process holds one or more resources that the next process in the chain is waiting for.

Detection and Recovery

Detecting deadlocks involves the use of algorithms that check for the existence of cycles in the resource-allocation graph. Recovery from deadlock can be done in one of several ways:

• Preemption: Temporarily taking resources away from one or more processes.
• Rollback: Returning the system to a safe state that is free of deadlock.
• Killing Processes: Abruptly terminating one or more processes involved in the deadlock.

Avoidance and Prevention

Deadlock avoidance and prevention strategies aim to ensure that at least one of the necessary conditions for deadlock does not occur. Techniques include:

• Resource Allocation Graph Algorithm: Dynamically checking the system to ensure that circular wait conditions do not hold.
• Banker's Algorithm: A strategy used in resource allocation that ensures a system remains in a safe state by simulating resource allocation for requested resources and determining if moving forward is safe.
• Lock Ordering: A preventative measure that involves imposing a total ordering on all resource types and ensuring that each process requests resources in an increasing order of enumeration.

Examples

In real-world computing environments, deadlocks can occur in various scenarios, such as:

• Database systems where transactions require locks on multiple rows or tables.
• Operating systems where processes may require exclusive access to devices, files, or other resources.
• Multi-threaded applications where threads may wait indefinitely for locks held by other threads.

Conclusion

Deadlocks are a critical issue in the design and operation of concurrent systems. Understanding the conditions that lead to deadlocks and implementing strategies for detection, prevention, and recovery are essential for maintaining system stability and performance.

Deadlock Resources
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