Infinite Loop when automounting device mounted elsewhere

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

Hello,

I just ran into this issue:

  $ mkdir /boot
  $ mount /dev/mmc2.0 /boot
  $ ls /mnt/mmc2.0/
  mounted /dev/mmc2.0 on /boot
  mounted /dev/mmc2.0 on /boot
  mounted /dev/mmc2.0 on /boot
  [ Ad infinitum ]

This is more readily reproducible with barebox platforms where a separate
boot partition is default-mounted as /boot. In that case, accessing the
partition's corresponding /mnt entry will hang.

The hang can be traced back to fs/fs.c:follow_managed():

  while (managed = path->dentry->d_flags,
  	managed &= DCACHE_MANAGED_DENTRY,
  	managed != 0) {
	if (managed & DCACHE_MOUNTED) {
  		struct vfsmount *mounted = lookup_mnt(path);
  
  		if (mounted) {
  			dput(path->dentry);
  			if (need_mntput)
  				mntput(path->mnt);
  			path->mnt = mounted;
  			path->dentry = dget(mounted->mnt_root);
  			need_mntput = true;
  			continue;
  		}
  	}
  
  	/* Handle an automount point */
  	if (managed & DCACHE_NEED_AUTOMOUNT) {
  		ret = follow_automount(path, nd, &need_mntput);
  		if (ret < 0)
  			break;
  		continue;
  	}
  
	/* We didn't change the current path point */
  	break;
  }


The code attempts the follow_automount, which would succeed here,
but it doesn't actually mount the current mount point, so we never
enter the first if clause that would change path->dentry and lead
us out of the loop.

I am unsure at what layer this should be fixed. Any suggestions?

Cheers,
Ahmad

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