What four conditions are required for a deadlock?

The four Coffman conditions, all of which must hold simultaneously for a deadlock to be possible: mutual exclusion (resources are held exclusively, only one owner at a time, inherent to mutexes); hold-and-wait (a task holds at least one resource while waiting to acquire another); no preemption (a resource can't be forcibly taken from its holder, it's only released voluntarily); and circular wait (there exists a cycle of tasks where each is waiting for a resource the next task in the cycle holds). The reason this framing is useful is that breaking any single one of the four makes deadlock impossible, so every prevention technique maps to eliminating one condition: consistent lock ordering eliminates circular wait; acquiring all locks at once or releasing before acquiring eliminates hold-and-wait; timeouts with release-and-retry effectively break the no-preemption/indefinite-wait condition by making the wait give up; and using lock-free structures or not sharing the resource eliminates mutual exclusion. In practice circular wait is the easiest to attack (via lock ordering), which is why that's the go-to fix.