alt Hacker NewsThe Level<> abstraction is a really neat way to have your cake and eat it too: you only need a consistent arbitrary order to avoid deadlocks, but the order can have performance consequences when some locks are more coarse than others.
But the example seems backwards to me: unless every callsite that locks any item always locks the big global lock first (probably not true, because if you serialize all item access on a global lock then a per-item lock serves no purpose...), aren't you begging for priority inversions by acquiring the big global lock before you acquire the item lock?
My only gripe is missing the obvious opportunity for Ferengi memes ("rules of acquisition") :D :D
There’s no global lock. There’s a linear MutexKey<N> that a lock of Level >= N has to be acquired with. Aquiring it consumes MutexKey<N> and hands you back MutexKey<Level+1> where Level is the N of the level you’re locking.
There’s no priority inversion possible because locks can only ever be held in decreasing orders of priority - you can’t acquire a low priority lock and then a high priority lock since your remaining MutexKey won’t have the right level.