Deadlocks, Livelocks, & Race Conditions

A Deadlock is a state in which two or more concurrent execution threads (or distributed transactions) are permanently blocked because each holds a resource that another needs, and none can proceed.

In 1971, computer scientist Edward G. Coffman Jr. proved that a deadlock can occur if and only if all four of the following conditions hold simultaneously:

1. Mutual Exclusion: Resources cannot be shared; only one thread can hold a resource at any given moment.
2. Hold and Wait: A thread holding at least one resource is actively waiting to acquire additional resources held by other threads.
3. No Preemption: Resources cannot be forcibly confiscated from a thread; a resource can only be released voluntarily by the holding thread.
4. Circular Wait: A closed chain of threads exists such that T_1 waits for a resource held by T_2, T_2 waits for T_3, and T_n waits for a resource held by T_1.

To permanently prevent deadlocks in any software architecture, you must break at least one of these four conditions.