A deadlock in computer science and operating systems is a situation where two or more processes are unable to proceed because each is waiting for the other to release a resource. In a deadlock, a set of processes is blocked because each process is holding a resource and waiting for another resource acquired by some other process in the set. As a result, none of the processes can move forward.
Deadlocks can occur in systems that use multiple processes or threads and involve resource allocation. The conditions necessary for a deadlock to occur are often described using the following four conditions, known as the Coffman conditions:
Mutual Exclusion: At least one resource must be held in a non-sharable mode, meaning that only one process can use the resource at a time.
Hold and Wait: A process must be holding at least one resource and waiting to acquire additional resources that are currently held by other processes.
No Preemption: Resources cannot be forcibly taken away from a process; a process must release its held resources voluntarily.
Circular Wait: A set of processes exists, each of which is waiting for a resource held by another process in the set, creating a circular chain of waiting.
When these four conditions are met, a deadlock can occur. Deadlocks can have serious consequences in a system, leading to resource wastage and a loss of system efficiency.
To prevent and manage deadlocks, various approaches are used, including deadlock prevention, deadlock avoidance, and deadlock detection with recovery. These techniques involve careful resource allocation, monitoring, and, if necessary, the implementation of mechanisms to break or avoid deadlocks.