This is the key sticky bit. With hardware support for pre-emption it can work. I saw a very clever hack on a CPU for doing T1 processing (the telecoms protocol) which basically had four tasks that 'could' run and the engineer used the hardware data watch breakpoint (up to four addresses) which would interrupt to a vector when data changed at that address. This combined with the serdes interrupt for line data basically allowed everyone to 'sleep' most of the time.

Clever system but very very very difficult to maintain.