Deadlock conditions

From APIDesign

Contents 1 Mutual exclusion 2 Hold and wait 3 Cannot Take Away a Resource 4 Resource waiting or circular wait

Mutual exclusion

The resources involved must be exclusive. Once a thread gets on hold of a resource, nobody else can use it. Otherwise the concurrent threads would not be prevented from using the resource.

Hold and wait

There must be a way to request another resource while already holding some. If a thread can only request resource(s) while holding none, a deadlock cannot be created.

Cannot Take Away a Resource

Nobody can take away a resource from a thread. If there was a supervisor, that could take the resource away, it could effectively solve all deadlocks by detecting them (which is easy) and then choosing a thread to loose its resource while letting others proceed.

Resource waiting or circular wait

A circular chain of processes, with each process holding resources which are currently being requested by the next process in the chain, cannot exist. If it does, the cycle theorem (which states that "a cycle in the resource graph is necessary for deadlock to occur") indicated that deadlock could occur.

four necessary and also sufficient conditions