A deadlock is a situation in computer science and computing where two or more processes or threads remain in a waiting state because each is waiting for a resource held by another process or thread. This results in none of the involved processes or threads being able to proceed, causing a complete halt of the affected parts of the system.
Conditions for a Deadlock
For a deadlock to occur, four conditions, known as Coffman conditions, must hold simultaneously:
- Mutual Exclusion: The resources involved can only be used by one process or thread at a time.
- Hold and Wait: A process or thread that is holding at least one resource is waiting to acquire additional resources that are currently being held by other processes or threads.
- No Preemption: Resources cannot be forcibly taken from the holding processes or threads; they can only be released voluntarily.
- Circular Wait: There exists a set of two or more processes or threads, each of which is waiting for a resource that is held by the next process in the chain.
Examples
A simple example of a deadlock is the classic problem involving two processes, each needing access to two resources:
- Process A: Holds Resource 1 and waits for Resource 2.
- Process B: Holds Resource 2 and waits for Resource 1.
Strategies to Avoid and Resolve Deadlocks
- Avoidance: Algorithms like the Banker's Algorithm can ensure that the system never enters a deadlock state.
- Detection: Systems can implement mechanisms to detect deadlocks and take actions to resolve them, such as terminating one of the involved processes.
- Prevention: Implementing protocols and rules to ensure that at least one of the Coffman conditions cannot hold.
- Resolution: Once a deadlock is detected, various strategies can be used to resolve it, such as rolling back processes or releasing resources.
Deadlocks are a significant issue in system and software development, especially in parallel and distributed processing, and require careful planning and control to avoid and manage them effectively.
