How is spaghetti code to refactor some common aspect into a reusable method with clearly defined boundaries?
I took some time to refactor it all to not use recursive locks, instead having a clean boundary between internal (non-locking) methods and external methods which did the locking and called the internal ones.
Not only did our locking issues go away immediately, it lead to a much simpler developer experience since there was never any question if this new function needed to lock or not etc.
I've not written that heavy multi-threaded code since, but my takeaway from the experience was that requiring recursive locks was a sign of poor design.
Instead of one "public" function call resulting in locking the same lock recursively 4-5 times as the "public" function called other functions and so on, a single locking operation was done at the boundary.