On Fri, Mar 06, 2020 at 08:37:05PM +0000, Al Viro wrote:

You are misreading mntput_no_expire(), BTW - your get_mount() can
bloody well race with umount(2), hitting the moment when we are done
figuring out whether it's busy but hadn't cleaned ->mnt_ns (let alone
set MNT_DOOMED) yet.  If somebody calls umount(2) on a filesystem that
is not mounted anywhere else, they are not supposed to see the sucker
return 0 until the filesystem is shut down.  You break that.

While we are at it, d_alloc_parallel() requires i_rwsem on parent held
at least shared.