Index: sys/fs/autofs/autofs.h
===================================================================
--- sys/fs/autofs/autofs.h
+++ sys/fs/autofs/autofs.h
@@ -41,6 +41,7 @@
 
 extern int autofs_debug;
 extern int autofs_mount_on_stat;
+extern int autofs_obsolete_lookup;
 
 #define	AUTOFS_DEBUG(X, ...)						\
 	do {								\
Index: sys/fs/autofs/autofs.c
===================================================================
--- sys/fs/autofs/autofs.c
+++ sys/fs/autofs/autofs.c
@@ -148,6 +148,11 @@
 TUNABLE_INT("vfs.autofs.interruptible", &autofs_interruptible);
 SYSCTL_INT(_vfs_autofs, OID_AUTO, interruptible, CTLFLAG_RWTUN,
     &autofs_interruptible, 1, "Allow requests to be interrupted by signal");
+int autofs_obsolete_lookup = 0;
+TUNABLE_INT("vfs.autofs.obsolete_lookup", &autofs_obsolete_lookup);
+SYSCTL_INT(_vfs_autofs, OID_AUTO, obsolete_lookup, CTLFLAG_RWTUN,
+    &autofs_obsolete_lookup, 0, "Use the old way of interacting "
+    "with lookup(9)");
 
 int
 autofs_init(struct vfsconf *vfsp)
Index: sys/fs/autofs/autofs_vnops.c
===================================================================
--- sys/fs/autofs/autofs_vnops.c
+++ sys/fs/autofs/autofs_vnops.c
@@ -257,23 +257,33 @@
 			return (error);
 
 		if (newvp != NULL) {
-			error = VOP_LOOKUP(newvp, ap->a_vpp, ap->a_cnp);
+			if (autofs_obsolete_lookup) {
+				error = VOP_LOOKUP(newvp, ap->a_vpp, ap->a_cnp);
 
-			/*
-			 * Instead of figuring out whether our vnode should
-			 * be locked or not given the error and cnp flags,
-			 * just "copy" the lock status from vnode returned
-			 * by mounted filesystem's VOP_LOOKUP().  Get rid
-			 * of that new vnode afterwards.
-			 */
-			lock_flags = VOP_ISLOCKED(newvp);
-			if (lock_flags == 0) {
-				VOP_UNLOCK(dvp, 0);
-				vrele(newvp);
+				/*
+				 * Instead of figuring out whether our vnode should
+				 * be locked or not given the error and cnp flags,
+				 * just "copy" the lock status from vnode returned
+				 * by mounted filesystem's VOP_LOOKUP().  Get rid
+				 * of that new vnode afterwards.
+				 */
+				lock_flags = VOP_ISLOCKED(newvp);
+				if (lock_flags == 0) {
+					VOP_UNLOCK(dvp, 0);
+					vrele(newvp);
+				} else {
+					vput(newvp);
+				}
+				return (error);
 			} else {
+				/*
+				 * The target filesystem got automounted.
+				 * Let the lookup(9) go around with the same
+				 * path component.
+				 */
 				vput(newvp);
+				return (ERELOOKUP);
 			}
-			return (error);
 		}
 	}
 
Index: sys/fs/nullfs/null_vnops.c
===================================================================
--- sys/fs/nullfs/null_vnops.c
+++ sys/fs/nullfs/null_vnops.c
@@ -429,6 +429,16 @@
 				*ap->a_vpp = vp;
 		}
 	}
+
+	if (error == ERELOOKUP) {
+		/*
+		 * Nullfs is not supposed to cross mount points, and returning
+		 * ERELOOKUP here would end up triggering an inifinite lookup
+		 * loop anyway; explicitly disallow this case.
+		 */
+		error = EOPNOTSUPP;
+	}
+
 	return (error);
 }
 
Index: sys/fs/unionfs/union_vnops.c
===================================================================
--- sys/fs/unionfs/union_vnops.c
+++ sys/fs/unionfs/union_vnops.c
@@ -174,6 +174,15 @@
 	if (udvp != NULLVP) {
 		uerror = VOP_LOOKUP(udvp, &uvp, cnp);
 
+		if (uerror == ERELOOKUP) {
+			/*
+			 * Unionfs is not supposed to cross mount points, and returning
+			 * ERELOOKUP here would end up triggering an inifinite lookup
+			 * loop anyway; explicitly disallow this case.
+			 */
+			uerror = EOPNOTSUPP;
+		}
+
 		if (uerror == 0) {
 			if (udvp == uvp) {	/* is dot */
 				vrele(uvp);
@@ -213,6 +222,15 @@
 
 		lerror = VOP_LOOKUP(ldvp, &lvp, cnp);
 
+		if (lerror == ERELOOKUP) {
+			/*
+			 * Unionfs is not supposed to cross mount points, and returning
+			 * ERELOOKUP here would end up triggering an inifinite lookup
+			 * loop anyway; explicitly disallow this case.
+			 */
+			lerror = EOPNOTSUPP;
+		}
+
 		cnp->cn_nameiop = nameiop;
 		if (udvp != NULLVP && (uerror == 0 || uerror == EJUSTRETURN))
 			cnp->cn_flags = cnflagsbk;
Index: sys/kern/vfs_lookup.c
===================================================================
--- sys/kern/vfs_lookup.c
+++ sys/kern/vfs_lookup.c
@@ -495,6 +495,7 @@
 	int rdonly;			/* lookup read-only flag bit */
 	int error = 0;
 	int dpunlocked = 0;		/* dp has already been unlocked */
+	int relookup = 0;		/* do not consume the path component */
 	struct componentname *cnp = &ndp->ni_cnd;
 	int lkflags_save;
 	int ni_dvp_unlocked;
@@ -732,6 +733,13 @@
 #ifdef NAMEI_DIAGNOSTIC
 		printf("not found\n");
 #endif
+		if (error == ERELOOKUP) {
+			vref(dp);
+			ndp->ni_vp = dp;
+			error = 0;
+			relookup = 1;
+			goto good;
+		}
 		if ((error == ENOENT) &&
 		    (dp->v_vflag & VV_ROOT) && (dp->v_mount != NULL) &&
 		    (dp->v_mount->mnt_flag & MNT_UNION)) {
@@ -777,6 +785,8 @@
 		goto success;
 	} else
 		cnp->cn_lkflags = lkflags_save;
+
+good:
 #ifdef NAMEI_DIAGNOSTIC
 	printf("found\n");
 #endif
@@ -856,6 +866,14 @@
 	 */
 	KASSERT((cnp->cn_flags & ISLASTCN) || *ndp->ni_next == '/',
 	    ("lookup: invalid path state."));
+	if (relookup) {
+		relookup = 0;
+		if (ndp->ni_dvp != dp)
+			vput(ndp->ni_dvp);
+		else
+			vrele(ndp->ni_dvp);
+		goto dirloop;
+	}
 	if (*ndp->ni_next == '/') {
 		cnp->cn_nameptr = ndp->ni_next;
 		while (*cnp->cn_nameptr == '/') {
Index: sys/sys/errno.h
===================================================================
--- sys/sys/errno.h
+++ sys/sys/errno.h
@@ -190,6 +190,7 @@
 #define	EJUSTRETURN	(-2)		/* don't modify regs, just return */
 #define	ENOIOCTL	(-3)		/* ioctl not handled by this layer */
 #define	EDIRIOCTL	(-4)		/* do direct ioctl in GEOM */
+#define	ERELOOKUP	(-5)		/* retry the directory lookup */
 #endif
 
 #endif