Bug in the unmounting code

Hi,

I have noticed that there is a bug in unmounting code which could make 
filesystem unmountable when its parent filesystem has been forcefully 
unmounted. Following is quick way to reproduce the problem:

[sobomax_at_pioneer ~]$ sudo mkdir -p /tmp/1/2
[sobomax_at_pioneer ~]$ sudo mkdir /tmp/3
[sobomax_at_pioneer ~]$ sudo mount_nullfs /tmp/1 /tmp/3
[sobomax_at_pioneer ~]$ sudo mount_nullfs /tmp/1 /tmp/3/2
[sobomax_at_pioneer ~]$ sudo umount -f /tmp/3
[sobomax_at_pioneer ~]$ sudo mount -v
/tmp/1 on /tmp/3/2 (nullfs, local, fsid 03ff000202000000)
[sobomax_at_pioneer ~]$ sudo umount 03ff000202000000
umount: unmount of /tmp/3/2 failed: No such file or directory
umount: retrying using path instead of file system ID
umount: unmount of /tmp/3/2 failed: No such file or directory

Investigation has revealed that in this case vn_lock() call fails with 
ENOENT due to the following piece of code:

vn_lock()
[...]
                 if (error == 0 && vp->v_iflag & VI_DOOMED &&
                     (flags & LK_RETRY) == 0) {
                         VOP_UNLOCK(vp, 0, td);
                         error = ENOENT;
                         break;
                 }
[...]

Addition of LK_RETRY flag fixed the problem, but my knowledge of VFS is 
quite limited so that I would appreciate if somebody could verify that 
the fix below won't have any undesirable effects.

-Maxim

--- vfs_mount.c 2007/04/21 01:40:53     1.1
+++ vfs_mount.c 2007/04/21 01:41:09
_at__at_ -1155,7 +1155,7 _at__at_
                 mnt_gen_r = mp->mnt_gen;
                 VI_LOCK(coveredvp);
                 vholdl(coveredvp);
-               error = vn_lock(coveredvp, LK_EXCLUSIVE | LK_INTERLOCK, td);
+               error = vn_lock(coveredvp, LK_EXCLUSIVE | LK_INTERLOCK | 
LK_RETRY, td);
                 vdrop(coveredvp);
                 /*
                  * Check for mp being unmounted while waiting for the