fixing "umount -f" for the NFS client

[Konstantin Belousov]

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.
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