Individually necessary and jointly sufficient conditions for deadlock Ī deadlock situation on a resource can arise if and only if all of the following conditions occur simultaneously in a system:
If a process remains indefinitely unable to change its state because resources requested by it are being used by another process that itself is waiting, then the system is said to be in a deadlock. In an operating system, a deadlock occurs when a process or thread enters a waiting state because a requested system resource is held by another waiting process, which in turn is waiting for another resource held by another waiting process.
Deadlocks are a common problem in multiprocessing systems, parallel computing, and distributed systems, because in these contexts systems often use software or hardware locks to arbitrate shared resources and implement process synchronization. In concurrent computing, deadlock is any situation in which no member of some group of entities can proceed because each waits for another member, including itself, to take action, such as sending a message or, more commonly, releasing a lock.
The deadlock can be resolved by breaking the symmetry. A deadlock occurs when all processes lock the resource simultaneously (black lines). Deadlocks that occupy nonpreemptable resources are hard to deal with.Four processes (blue lines) compete for one resource (grey circle), following a right-before-left policy. Reallocating resources can determine deadlocks that involve preemptable resources. For instance, CD resources are not preemptable at an arbitrary instant. Alternatively, a nonpreemptable resource is one that cannot be taken away from process (with no causing ill effect). Memory is an instance of a preemptable resource. A preemptable resource is one that can be taken away from the process with no ill effects.