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.
