Function zap_lockdir_impl

freebsd/contrib/openzfs/module/zfs/zap_micro.c:507–580

* This routine "consumes" the caller's hold on the dbuf, which must * have the specified tag. */

 * have the specified tag.
 */
static int
zap_lockdir_impl(dmu_buf_t *db, void *tag, dmu_tx_t *tx,
    krw_t lti, boolean_t fatreader, boolean_t adding, zap_t **zapp)
{
	ASSERT0(db->db_offset);
	objset_t *os = dmu_buf_get_objset(db);
	uint64_t obj = db->db_object;
	dmu_object_info_t doi;

	*zapp = NULL;

	dmu_object_info_from_db(db, &doi);
	if (DMU_OT_BYTESWAP(doi.doi_type) != DMU_BSWAP_ZAP)
		return (SET_ERROR(EINVAL));

	zap_t *zap = dmu_buf_get_user(db);
	if (zap == NULL) {
		zap = mzap_open(os, obj, db);
		if (zap == NULL) {
			/*
			 * mzap_open() didn't like what it saw on-disk.
			 * Check for corruption!
			 */
			return (SET_ERROR(EIO));
		}
	}

	/*
	 * We're checking zap_ismicro without the lock held, in order to
	 * tell what type of lock we want.  Once we have some sort of
	 * lock, see if it really is the right type.  In practice this
	 * can only be different if it was upgraded from micro to fat,
	 * and micro wanted WRITER but fat only needs READER.
	 */
	krw_t lt = (!zap->zap_ismicro && fatreader) ? RW_READER : lti;
	rw_enter(&zap->zap_rwlock, lt);
	if (lt != ((!zap->zap_ismicro && fatreader) ? RW_READER : lti)) {
		/* it was upgraded, now we only need reader */
		ASSERT(lt == RW_WRITER);
		ASSERT(RW_READER ==
		    ((!zap->zap_ismicro && fatreader) ? RW_READER : lti));
		rw_downgrade(&zap->zap_rwlock);
		lt = RW_READER;
	}

	zap->zap_objset = os;

	if (lt == RW_WRITER)
		dmu_buf_will_dirty(db, tx);

	ASSERT3P(zap->zap_dbuf, ==, db);

	ASSERT(!zap->zap_ismicro ||
	    zap->zap_m.zap_num_entries <= zap->zap_m.zap_num_chunks);
	if (zap->zap_ismicro && tx && adding &&
	    zap->zap_m.zap_num_entries == zap->zap_m.zap_num_chunks) {
		uint64_t newsz = db->db_size + SPA_MINBLOCKSIZE;
		if (newsz > MZAP_MAX_BLKSZ) {