holdandwait

Hold-and-wait is a deadlock condition in concurrent systems in which a process that holds at least one resource is waiting to acquire additional resources that are currently held by other processes. This situation can prevent progress when multiple resources are involved and processes hold some while waiting for others.

In resource-sharing environments, a process may hold a mutex, file handle, or memory block while it waits for another resource. If several processes do this in a cycle, none can proceed, leading to a deadlock. Hold-and-wait is one of the four Coffman conditions for deadlock, along with mutual exclusion, no preemption, and circular wait.

Prevention and handling strategies include requiring a process to request all needed resources at once (all-or-nothing allocation), or forcing resources to be acquired in a fixed global order to avoid cycles. Other approaches include allowing preemption of resources, using timeouts or deadlock detection with rollback, and applying resource hierarchy or scheduling policies that reduce the likelihood of hold-and-wait. In practice, some systems tolerate hold-and-wait to improve utilization, accepting occasional deadlocks that are resolved by the chosen prevention or detection mechanism.

Example: Two processes, P1 holds resource A and waits for B, while P2 holds B and waits for A. Neither can proceed, creating a deadlock. Breaking hold-and-wait can be achieved by ordering resource acquisition or by requiring processes to obtain all necessary resources before continuing.