What are the requirements for a deadlock situation?

In general the following requirements for a deadlock can be identified:

• Mutual exclusion: There is a resource which can be accessed only by one thread at any point in time.
• Resource holding: While having locked one resource, the thread tries to acquire another lock on some other exclusive resource.
• No preemption: There is no mechanism, which frees the resource if one thread holds the lock for a specific period of time.
• Circular wait: During runtime a constellation occurs in which two (or more) threads are each waiting on the other thread to free a resource that it has locked.