CIRC_ADD
#define CIRC_NEXT(val, size) CIRC_ADD(val, 1, size)
#define CIRC_PREV(val, size) CIRC_ADD(val, -1, size)
CIRC_ADD(req->clear_tail,
#define CIRC_NEXT(val, size) CIRC_ADD(val, 1, size)
#define CIRC_PREV(val, size) CIRC_ADD(val, -1, size)
cb->head = CIRC_ADD(cb->head, LOG_SIZE, 1);
cb->tail = CIRC_ADD(cb->tail, LOG_SIZE, ret);