[LU-10824] ll_umount being improper use of cached mountpoint from mount time

Related — Thu, 6 Jan 2022 22:00:23 +0000

Al Viro chimed in complaining about patch from ~~LU-1882~~ - http://review.whamcloud.com/20061

Basically the problem is we cache sbi->ll_mnt.mnt as

»       sbi->ll_mnt.mnt = current->fs->root.mnt;

This is prone to many errors, in particular: did not get reference count, so this mount struct might disappear, it does not take bind mounts into account not mount --move.
Might result in kernel panics in certain conditions.

Overall original patch is trying to "fix" something that's not broken in the kernel

If somebody does stat mountpoint in a tight loop, you will get -EBUSY there.
No matter what. What's more, you might very well get it after some check in
->umount_begin() has told you that at the moment the thing you were trying to
umount was not busy - just before the next stat hits.

The patch needs to be reworked and we just need to add some extra retrying logic in the test framework instead I guess.

Whamcloud Community JIRA

[LU-10824] ll_umount being improper use of cached mountpoint from mount time