diff -urBN /usr/src.orig/sys/fs/unionfs/union_vfsops.c /usr/src/sys/fs/unionfs/union_vfsops.c
--- /usr/src.orig/sys/fs/unionfs/union_vfsops.c	2011-04-18 15:06:07.000000000 +0900
+++ /usr/src/sys/fs/unionfs/union_vfsops.c	2011-05-09 18:41:08.000000000 +0900
@@ -326,7 +326,10 @@
 static int
 unionfs_unmount(struct mount *mp, int mntflags)
 {
+	struct mount   *pmp;
+	struct vnode   *pvp;
 	struct unionfs_mount *ump;
+	struct unionfs_mount *pump;
 	int		error;
 	int		num;
 	int		freeing;
@@ -335,11 +338,32 @@
 	UNIONFSDEBUG("unionfs_unmount: mp = %p\n", (void *)mp);
 
 	ump = MOUNTTOUNIONFSMOUNT(mp);
+	pvp = NULLVP;
 	flags = 0;
+	error = 0;
+	num = 0;
 
 	if (mntflags & MNT_FORCE)
 		flags |= FORCECLOSE;
 
+	/* get parent vnode */
+	pmp = ump->um_rootvp->v_mountedhere;
+	if (pmp != NULL) {
+		if ((error = VFS_ROOT(pmp, LK_EXCLUSIVE, &pvp)) != 0)
+			return (error);
+		if (pvp->v_op != &unionfs_vnodeops ||
+		    pmp->mnt_vnodecovered != ump->um_rootvp ||
+		    (UNIONFSVPTOLOWERVP(pvp) != ump->um_rootvp &&
+		     UNIONFSVPTOUPPERVP(pvp) != ump->um_rootvp)) {
+			vput(pvp);
+			pvp = NULLVP;
+			pmp = NULL;
+		}
+	}
+
+	if (pvp != NULLVP)
+		VOP_UNLOCK(pvp, 0);
+
 	/* vflush (no need to call vrele) */
 	for (freeing = 0; (error = vflush(mp, 1, flags, curthread)) != 0;) {
 		num = mp->mnt_nvnodelistsize;
@@ -348,6 +372,46 @@
 		freeing = num;
 	}
 
+	UNIONFSDEBUG("unionfs_unmount: vnodesize=%d, parent=%p\n", num, pvp);
+
+	/* check */
+	if (pvp != NULLVP) {
+		vn_lock(pvp, LK_EXCLUSIVE | LK_RETRY);
+		if (error == (EBUSY) && num == 1) {
+			/* reconnect unionfs tree */
+			pump = MOUNTTOUNIONFSMOUNT(pmp);
+			if (pump->um_lowervp == ump->um_rootvp) {
+				vput(pump->um_lowervp);
+				vref(ump->um_lowervp);
+				vn_lock(ump->um_lowervp, LK_EXCLUSIVE|LK_RETRY);
+				pump->um_lowervp = ump->um_lowervp;
+				UNIONFSVPTOLOWERVP(pvp) = ump->um_lowervp;
+			}
+			else {
+				vput(pump->um_uppervp);
+				vref(ump->um_uppervp);
+				vn_lock(ump->um_uppervp, LK_EXCLUSIVE|LK_RETRY);
+				pump->um_uppervp = ump->um_uppervp;
+				UNIONFSVPTOUPPERVP(pvp) = ump->um_uppervp;
+			}
+
+			/* fix mount tree */
+			MNT_ILOCK(pmp);
+			vrele(pmp->mnt_vnodecovered);
+			vref(mp->mnt_vnodecovered);
+			pmp->mnt_vnodecovered = mp->mnt_vnodecovered;
+			pmp->mnt_vnodecovered->v_mountedhere = pmp;
+			mp->mnt_vnodecovered = NULLVP;
+			MNT_IUNLOCK(pmp);
+
+			/* retry vflush */
+			error = vflush(mp, 1, flags, curthread);
+			if (error)
+				panic("unionfs_unmount: invalid tree.");
+		}
+		vput(pvp);
+	}
+
 	if (error)
 		return (error);
 
diff -urBN /usr/src.orig/sys/kern/vfs_mount.c /usr/src/sys/kern/vfs_mount.c
--- /usr/src.orig/sys/kern/vfs_mount.c	2011-04-18 15:05:56.000000000 +0900
+++ /usr/src/sys/kern/vfs_mount.c	2011-05-09 17:50:16.000000000 +0900
@@ -1365,7 +1365,8 @@
 	TAILQ_REMOVE(&mountlist, mp, mnt_list);
 	mtx_unlock(&mountlist_mtx);
 	if (coveredvp != NULL) {
-		coveredvp->v_mountedhere = NULL;
+		if (coveredvp->v_mountedhere == mp)
+			coveredvp->v_mountedhere = NULL;
 		vput(coveredvp);
 	}
 	vfs_event_signal(NULL, VQ_UNMOUNT, 0);