If I read correctly, you only increment the epoch when the active bits for the current epoch are clear. As long as your stalled thread doesn't clear its active bit at an unsafe time, the epoch will stall with it and you'll be ok.

Not entirely sure how a thread is supposed to set its active bit without racing with a guy who just noticed that all active bits are clear. I guess that's why it says "try to increment the epoch" and not "increment it". i.e. it can fail. EDIT: I guess this is why you go two epoch units back and not one. That, combined with the idea that everybody only goes forward in time, would probably do it.