Re: [PATCH] vfs: introduce UMOUNT_WAIT which waits for umount completion

[Al Viro]

On Fri, Sep 15, 2017 at 04:29:11PM -0700, Jaegeuk Kim wrote:

> The mntput() in delayed_fput() is the last function call. So before that moment,
> sys_umount() may see mnt_get_count() as 2, so it avoids EBUSY condition. I'm not
> sure why it check over 2 tho.

Because it has just grabbed a reference itself, in addition to the one that keeps
the damn thing alive (due to being mounted).  So it bloody well should have
triggered -EBUSY, if they refer to the same vfsmount.