work_next_color
WARN_ON_ONCE(work_color != work_next_color(pwq->work_color));
next_color = work_next_color(wq->work_color);
wq->flush_color != work_next_color(wq->work_color));
wq->flush_color = work_next_color(wq->flush_color);
wq->work_color = work_next_color(wq->work_color);