mac_srs_pick_chain
if ((head = mac_srs_pick_chain(mac_srs, &tail, &sz, &cnt)) == NULL) {
mblk_t *admit_head = mac_srs_pick_chain(mac_srs, &admit_tail,
head = mac_srs_pick_chain(mac_srs, &tail, &chain_sz, &chain_ct);
head = mac_srs_pick_chain(mac_srs, &tail, &chain_sz, &chain_ct);