Dödlåsproblem

Dödlåsproblem, also known as deadlock in English, is a situation in computer science and other fields where two or more processes or entities are unable to proceed because each is waiting for the other to release a resource. This creates a circular dependency, preventing any of the involved parties from making progress.

In computing, deadlocks commonly occur in concurrent systems where multiple processes share resources such as memory, files, or locks. A process may acquire a lock on one resource and then request a lock on another resource that is currently held by another process. If this second process simultaneously holds a lock on the first resource and is waiting for the resource the first process is holding, a deadlock arises.

The four conditions necessary for a deadlock to occur, often referred to as the Coffman conditions, are mutual exclusion, hold and wait, no preemption, and circular wait. Mutual exclusion means that at least one resource must be held in a non-sharable mode. Hold and wait means that a process must be holding at least one resource and waiting to acquire additional resources that are held by other processes. No preemption means that resources cannot be forcibly taken away from a process holding them. Circular wait means that a set of processes {P0, P1, ..., Pn} must exist such that P0 is waiting for a resource held by P1, P1 is waiting for a resource held by P2, and so on, until Pn is waiting for a resource held by P0.

Strategies for handling deadlocks include deadlock prevention, deadlock avoidance, and deadlock detection and recovery. Deadlock prevention involves designing the system so that at least one of the four Coffman conditions can never hold. Deadlock avoidance involves dynamically checking resource allocation to ensure that a deadlock will not occur. Deadlock detection and recovery involves detecting when a deadlock has occurred and then taking steps to resolve it, such as terminating processes or preempting resources.