Re: NFSv4 cannot unmount ESTALE directories (in some cases).

[Al Viro <viro@ZenIV.linux.org.uk>]

On Mon, Jan 21, 2013 at 01:48:59PM +1100, NeilBrown wrote:
> 
> If you use NFSv4 to "mount server:/foo/bar /mnt", then "rm -r" /foo/bar on the
> server, then accesses to /mnt will naturally return ESTALE.
> 
> Unfortunately "umount /mnt" will also return ESTALE and leave the stale
> directory mounted.  Adding "-l" or "-f" to "umount" doesn't help.
> 
> The problem is that nfs_lookup_revalidate fails.  As the mountpoint is never
> not accessed by a lookup (after the initial mount) it seems a bit pointless
> calling d_revalidate in this case ... by maybe not.
> 
> I can make the problem go away by testing for LOOKUP_JUMP and having
> nfs_lookup_revalidate never fail if that flag it set (for a directory).

> However I cannot easily tell if this is an elegant solution of an ugly hack,

The latter.  Definitely.

> and am hoping that someone who understands revalidation and LOOKUP_JUMPED
> better than I (who only discovered the latter today) could provide advice.
> 
> Al?  Trond?  Should  I make this into a formal patch submission, or is there
> a better way?

I really suspect that mountpoint crossing on umount ought to be done
differently.  I'll need to play with possible variants a bit before I can
offer any replacement though...