chains_xa
#define chains_xa(chains) ((chains)->chains_xa)
err = xa_alloc(&chains_xa(chains), &chain_s->xa_idx, chain_s,
struct xarray chains_xa;
xa_erase(&chains_xa(chains), chain->xa_idx);
xa_for_each(&chains_xa(chains), idx, chain_s) {
xa_init(&chains_xa(chains));
xa_destroy(&chains_xa(chains));