Dödlås

Dödlås, or deadlock, is a state in which no progress can be made because multiple actors hold resources the others need to continue. In computing, dödlås describes a situation where two or more processes are blocked forever, each waiting for a resource held by another.

A classic description involves four conditions that Coffman outlined as typically necessary for a deadlock: mutual exclusion (resources are non-shareable while held), hold and wait (processes already holding resources can request more), no preemption (resources cannot be forcibly recovered), and circular wait (there is a closed chain of processes each waiting for the next resource held by the next process). When these conditions occur together, a deadlock can arise.

An example: Process A holds resource X and waits for Y, while Process B holds Y and waits for X. Neither can proceed, so both remain blocked.

Detection and resolution strategies vary. Prevention aims to eliminate one of the four conditions, for instance by imposing a strict order on resource acquisition or by preempting resources. Detection involves monitoring resource allocation graphs or wait-for graphs to identify cycles, after which one or more processes may be terminated, rolled back, or resources reallocated to break the cycle. Timeout mechanisms can also help by interrupting prolonged waits.

In practice, dödlås is a central concern in operating systems, databases, and distributed systems. Understanding its causes helps in designing concurrent algorithms, synchronization schemes, and resource management policies that minimize the risk of deadlocks. See also deadlock, concurrency control, and resource allocation.