fixing "umount -f" for the NFS client

[Rick Macklem]

Kostik wrote:
> On Wed, Aug 28, 2013 at 08:15:27PM -0400, Rick Macklem wrote:
> > I've been doing a little more testing of "umount -f" for NFS
> > mounts and they seem to be working unless some other process/thread
> > has busied the file system via vfs_busy().
> > 
> > Unfortunately, it is pretty easy to vfs_busy() the file system
> > by using a command like "df" that is stuck on the unresponsive
> > NFS server.
> > 
> > The problem seems to be that dounmount() msleep()s while
> > mnt_lockref != 0 before calling VFS_UNMOUNT().
> > 
> > If some call into the NFS client was done before this
> > while (mp->mnt_lockref) loop with msleep() in it, it
> > can easily kill off RPCs in progress. (It currently
> > does this in nfs_unmount() using the newnfs_nmcancelreqs()
> > call.
> > 
> > In summary:
> > - Would it be appropriate to add a new vfs_XXX method that
> >   dounmount() would call before the while() loop for the
> >   forced dismount case?
> >   (The default would be a no-op and I have no idea if any
> >    file system other than NFS would have a use for it?)
> >   Alternately, there could be a function pointer set non-NULL
> >   that would specifically be used by the NFS client for this.
> >   This would avoid adding a vfs_XXX() method, but would mean
> >   an NFS specific call ends up in the generic dounmount() code.
> > 
> > Anyone have comments on this?
> > 
> Yes, I do.  I agree with adding the pre-unmount vfs method.
> This seems to be the cleanest solution possible.
> 
I've attached a patch. It is also at
  http://people.freebsd.org/~rmacklem/forced-dism.patch
in case the attachment gets lost.
I don't really like doing the MNT_IUNLOCK(), MNT_ILOCK() before/after
the VFS_KILLIO() call, but I couldn't see any better way to do it and
it looks safe to do so, at least for the forced case.

I assume I would also need to bump __FreeBSD_version (and maybe VFS_VERSION?).

Please review it.

If anyone would like to test it, please do so.

Thanks, rick
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