FR_CLOSING
KASSERT((ff->ff_refcnt & FR_CLOSING) == 0);
if (__predict_false((ff->ff_refcnt & FR_CLOSING) != 0)) {
if (__predict_false((v & FR_CLOSING) != 0)) {
KASSERT((ff->ff_refcnt & FR_CLOSING) == 0);
atomic_or_uint(&ff->ff_refcnt, FR_CLOSING);
atomic_and_uint(&ff->ff_refcnt, ~FR_CLOSING);
if (ff->ff_refcnt & FR_CLOSING) {
#define FR_MASK (~FR_CLOSING) /* reference count */