Priority Inversion and Deadlock
A priority is a promise about who gets the core when two tasks want it. A lock is a mechanism that can break that promise, and the reason is structural rather than accidental: a task holding a lock has implicitly borrowed the urgency of every task that will ever wait for it, and the scheduler has no way of knowing that until someone actually waits. Until that moment the holder is running at its own priority, and the system is scheduling a task that is, in effect, on the critical path of something far more urgent.