ring_head
#define ring_posted(ring) (ring_head(ring) - ring_tail(ring))
unsigned long mseq = ring_head(ssk->tx_ring);
int id = ring_head(ssk->rx_ring);
ssk->recv_request_head = ring_head(ssk->rx_ring) + 1;
atomic_set(&ssk->tx_ring.credits, mseq_ack - ring_head(ssk->tx_ring) +
WARN_ON(ring_head(ssk->rx_ring) != ring_tail(ssk->rx_ring));
ring_head(ssk->tx_ring), ring_tail(ssk->tx_ring));
WARN_ON(ring_head(ssk->tx_ring) != ring_tail(ssk->tx_ring));
mseq = ring_head(ssk->tx_ring);