pstack of zfs recv semi-stalled between writes

root@jimoi:/root# pstack 16365
16365:  zfs recv -vue backup-adata/snapshots/nvpool/ROOT
 feedbc25 ioctl    (80b4548, 5a15, 80403e8, fe88efb0) + 15
 fe88f048 zfs_iter_snapshots (80b4548, 0, fe89cc4c, 8043b1c) + a9
 fe88f749 zfs_iter_children (80b4548, fe89cc4c, 8043b1c) + 26
 fe89ccc2 guid_to_name_cb (80b4548, 8043b1c) + 76
 fe88ef55 zfs_iter_filesystems (80b47c8, fe89cc4c, 8043b1c) + 9c
 fe88f765 zfs_iter_children (80b47c8, fe89cc4c, 8043b1c) + 42
 fe89ccc2 guid_to_name_cb (80b47c8, 8043b1c) + 76
 fe88ef55 zfs_iter_filesystems (80b4a48, fe89cc4c, 8043b1c) + 9c
 fe88f765 zfs_iter_children (80b4a48, fe89cc4c, 8043b1c) + 42
 fe89ccc2 guid_to_name_cb (80b4a48, 8043b1c) + 76
 fe89ce8d guid_to_name (80a9548, 8044fb8, d00f03c5, f9edef7f, 0) + 159
 fe8a1acc zfs_receive_one (80a9548, 0, 8047c4a, 0, 8047a44, 8046798, 8046660, 8046e98, ...) + b3a
 fe8a5530 zfs_receive_impl (80a9548, 8047c4a, 0, 8047a44, 0, 8046e98, 80a6f48, 80a6f68, ...) + 512
 fe8a4f45 zfs_receive_package (80a9548, 0, 8047c4a, 8047a44, 8047748, 80471f0, 80478fc, 9, ...) + 759
 fe8a54a4 zfs_receive_impl (80a9548, 8047c4a, 0, 8047a44, 0, 0, 0, 0, ...) + 486
 fe8a716b zfs_receive (80a9548, 8047c4a, 80a6fa8, 8047a44, 0, 0) + a9
 0805f214 zfs_do_receive (3, 8047b38) + 389
 08063339 main     (8047acc, fef585c8) + 231
 080566d7 _start_crt (4, 8047b34, fefd0d2f, 0, 0, 0) + 96
 080565aa _start   (4, 8047c3c, 8047c40, 8047c45, 8047c4a, 0) + 1a