On Mon, Apr 21, 2025 at 05:29:47PM +0100, Al Viro wrote:
> On Mon, Apr 21, 2025 at 09:56:20AM +0200, Christian Brauner wrote:
> > On Mon, Apr 21, 2025 at 04:35:09AM +0100, Al Viro wrote:
> > > Not since 8f2918898eb5 "new helpers: vfs_create_mount(), fc_mount()"
> > > back in 2018. Get rid of the dead checks...
> > >
> > > Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > > ---
> >
> > Good idea. Fwiw, I've put this into vfs-6.16.mount with some other minor
> > stuff. If you're keeping it yourself let me know.
>
> Not sure... I'm going through documenting the struct mount lifecycle/locking/etc.
> and it already looks like there will be more patches, but then some are going
> to be #fixes fodder.
>
> Example caught just a couple of minutes ago: do_lock_mount()
> if (beneath) {
> m = real_mount(mnt);
> read_seqlock_excl(&mount_lock);
> dentry = dget(m->mnt_mountpoint);
> read_sequnlock_excl(&mount_lock);
> } else {
> dentry = path->dentry;
> }
>
> inode_lock(dentry->d_inode);
> What's to prevent the 'beneath' case from getting mnt mount --move'd
> away *AND* the ex-parent from getting unmounted while we are blocked
> in inode_lock? At this point we are not holding any locks whatsoever
> (and all mount-related locks nest inside inode_lock(), so we couldn't
> hold them there anyway).
>
> Hit that race and watch a very unhappy umount...
If it gets unmounted or moved we immediately detect this in the next line:
if (beneath && (!is_mounted(mnt) || m->mnt_mountpoint != dentry)) {
