Index: sys/fs/unionfs/union_vfsops.c
===================================================================
--- sys/fs/unionfs/union_vfsops.c
+++ sys/fs/unionfs/union_vfsops.c
@@ -310,6 +310,30 @@
 		return (ENOENT);
 	}
 
+	/*
+	 * Specify that the covered vnode lock should remain held while
+	 * lookup() performs the cross-mount walk.  This prevents a lock-order
+	 * reversal between the covered vnode lock (which is also locked by
+	 * unionfs_lock()) and the mountpoint's busy count.  Without this,
+	 * unmount will lock the covered vnode lock (directly through the
+	 * covered vnode) and wait for the busy count to drain, while a
+	 * concurrent lookup will increment the busy count and then lock
+	 * the covered vnode lock (indirectly through unionfs_lock()).
+	 *
+	 * Note that we can't yet use this facility for the 'below' case
+	 * in which the upper vnode is the covered vnode, because that would
+	 * introduce a different LOR in which the cross-mount lookup would
+	 * effectively hold the upper vnode lock before acquiring the lower
+	 * vnode lock, while an unrelated lock operation would still acquire
+	 * the lower vnode lock before the upper vnode lock, which is the
+	 * order unionfs currently requires.
+	 */
+	if (!below) {
+		vn_lock(mp->mnt_vnodecovered, LK_EXCLUSIVE | LK_RETRY);
+		mp->mnt_vnodecovered->v_vflag |= VV_CROSSLOCK;
+		VOP_UNLOCK(mp->mnt_vnodecovered);
+	}
+
 	MNT_ILOCK(mp);
 	if ((lowermp->mnt_flag & MNT_LOCAL) != 0 &&
 	    (uppermp->mnt_flag & MNT_LOCAL) != 0)
@@ -362,6 +386,9 @@
 	if (error)
 		return (error);
 
+	vn_lock(mp->mnt_vnodecovered, LK_EXCLUSIVE | LK_RETRY);
+	mp->mnt_vnodecovered->v_vflag &= ~VV_CROSSLOCK;
+	VOP_UNLOCK(mp->mnt_vnodecovered);
 	vfs_unregister_upper(ump->um_lowervp->v_mount, &ump->um_lower_link);
 	vfs_unregister_upper(ump->um_uppervp->v_mount, &ump->um_upper_link);
 	free(ump, M_UNIONFSMNT);
Index: sys/kern/vfs_lookup.c
===================================================================
--- sys/kern/vfs_lookup.c
+++ sys/kern/vfs_lookup.c
@@ -107,11 +107,9 @@
 	file = ap->a_file;
 	line = ap->a_line;
 
-	if ((flags & LK_SHARED) == 0)
-		panic("invalid lock request for crossmp");
+	KASSERT((flags & (LK_SHARED | LK_NOWAIT)) == (LK_SHARED | LK_NOWAIT),
+	    ("%s: invalid lock request 0x%x for crossmp", __func__, flags));
 
-	WITNESS_CHECKORDER(&lk->lock_object, LOP_NEWORDER, file, line,
-	    flags & LK_INTERLOCK ? &VI_MTX(vp)->lock_object : NULL);
 	WITNESS_LOCK(&lk->lock_object, 0, file, line);
 	if ((flags & LK_INTERLOCK) != 0)
 		VI_UNLOCK(vp);
@@ -910,6 +908,8 @@
 	struct componentname *cnp = &ndp->ni_cnd;
 	int lkflags_save;
 	int ni_dvp_unlocked;
+	int crosslkflags;
+	bool crosslock;
 
 	/*
 	 * Setup: break out flag bits into variables.
@@ -1243,18 +1243,32 @@
 	 */
 	while (dp->v_type == VDIR && (mp = dp->v_mountedhere) &&
 	       (cnp->cn_flags & NOCROSSMOUNT) == 0) {
+		crosslock = ((dp->v_vflag & VV_CROSSLOCK) != 0);
+		crosslkflags = compute_cn_lkflags(mp, cnp->cn_lkflags,
+		    cnp->cn_flags);
+		if (__predict_false(crosslock) &&
+		    (crosslkflags & LK_EXCLUSIVE) != 0 &&
+		    VOP_ISLOCKED(dp) != LK_EXCLUSIVE) {
+			vn_lock(dp, LK_UPGRADE | LK_RETRY);
+			if (VN_IS_DOOMED(dp)) {
+				error = ENOENT;
+				goto bad2;
+			}
+		}
 		if (vfs_busy(mp, 0))
 			continue;
-		vput(dp);
+		if (__predict_true(!crosslock))
+			vput(dp);
 		if (dp != ndp->ni_dvp)
 			vput(ndp->ni_dvp);
 		else
 			vrele(ndp->ni_dvp);
 		vrefact(vp_crossmp);
 		ndp->ni_dvp = vp_crossmp;
-		error = VFS_ROOT(mp, compute_cn_lkflags(mp, cnp->cn_lkflags,
-		    cnp->cn_flags), &tdp);
+		error = VFS_ROOT(mp, crosslkflags, &tdp);
 		vfs_unbusy(mp);
+		if (__predict_false(crosslock))
+			vput(dp);
 		if (vn_lock(vp_crossmp, LK_SHARED | LK_NOWAIT))
 			panic("vp_crossmp exclusively locked or reclaimed");
 		if (error) {
Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -770,17 +770,22 @@
  *              +->F->D->E
  *
  * The lookup() process for namei("/var") illustrates the process:
- *  VOP_LOOKUP() obtains B while A is held
- *  vfs_busy() obtains a shared lock on F while A and B are held
- *  vput() releases lock on B
- *  vput() releases lock on A
- *  VFS_ROOT() obtains lock on D while shared lock on F is held
- *  vfs_unbusy() releases shared lock on F
- *  vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A.
- *    Attempt to lock A (instead of vp_crossmp) while D is held would
- *    violate the global order, causing deadlocks.
+ *  1. VOP_LOOKUP() obtains B while A is held
+ *  2. vfs_busy() obtains a shared lock on F while A and B are held
+ *  3. vput() releases lock on B
+ *  4. vput() releases lock on A
+ *  5. VFS_ROOT() obtains lock on D while shared lock on F is held
+ *  6. vfs_unbusy() releases shared lock on F
+ *  7. vn_lock() obtains lock on deadfs vnode vp_crossmp instead of A.
+ *     Attempt to lock A (instead of vp_crossmp) while D is held would
+ *     violate the global order, causing deadlocks.
  *
- * dounmount() locks B while F is drained.
+ * dounmount() locks B while F is drained.  Note that for stacked
+ * filesystems, D and B in the example above may be the same lock,
+ * which introdues potential lock order reversal deadlock between
+ * dounmount() and step 5 above.  These filesystems may avoid the LOR
+ * by setting VV_CROSSLOCK on the covered vnode so that lock B will
+ * remain held until after step 5.
  */
 int
 vfs_busy(struct mount *mp, int flags)
Index: sys/sys/vnode.h
===================================================================
--- sys/sys/vnode.h
+++ sys/sys/vnode.h
@@ -276,6 +276,7 @@
 #define	VV_FORCEINSMQ	0x1000	/* force the insmntque to succeed */
 #define	VV_READLINK	0x2000	/* fdescfs linux vnode */
 #define	VV_UNREF	0x4000	/* vunref, do not drop lock in inactive() */
+#define	VV_CROSSLOCK	0x8000	/* vnode lock is shared w/ root mounted here */
 
 #define	VMP_LAZYLIST	0x0001	/* Vnode is on mnt's lazy list */