Deadlock conditions
From APIDesign
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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
A thread can hold and wait indefinitely. If it could not, and for example would time out, the deadlock can be resolved by letting the timed out thread to give up and release all resources.
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.
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