> Which just means that waiting on C conditions is C times more expensive than
> waiting on 1 currently is.  That seems reasonable enough to me, and anyway,
> I would expect C to be fairly small.

Waking up a single thread (through notify()) is currently O(1) (since
it's unqueued from the head of the deque). Here, removing a thread
from a wait queue other than the one from which it was signalled is
O(waiting threads).