Index: head/sys/kern/vfs_mount.c
===================================================================
--- head/sys/kern/vfs_mount.c
+++ head/sys/kern/vfs_mount.c
@@ -1359,6 +1359,8 @@
 		vput(coveredvp);
 	}
 	vfs_event_signal(NULL, VQ_UNMOUNT, 0);
+	if (mp == rootdevmp)
+		rootdevmp = NULL;
 	vfs_mount_destroy(mp);
 	return (0);
 }
Index: head/sys/kern/vfs_mountroot.c
===================================================================
--- head/sys/kern/vfs_mountroot.c
+++ head/sys/kern/vfs_mountroot.c
@@ -95,6 +95,11 @@
  */
 struct vnode *rootvnode;
 
+/*
+ * Mount of the system's /dev.
+ */
+struct mount *rootdevmp;
+
 char *rootdevnames[2] = {NULL, NULL};
 
 struct mtx root_holds_mtx;
@@ -236,6 +241,7 @@
 	mtx_unlock(&mountlist_mtx);
 
 	*mpp = mp;
+	rootdevmp = mp;
 	set_rootvnode();
 
 	error = kern_symlinkat(td, "/", AT_FDCWD, "dev", UIO_SYSSPACE);
Index: head/sys/kern/vfs_subr.c
===================================================================
--- head/sys/kern/vfs_subr.c
+++ head/sys/kern/vfs_subr.c
@@ -3543,6 +3543,21 @@
     "");
 #endif
 
+static void
+unmount_or_warn(struct mount *mp)
+{
+	int error;
+
+	error = dounmount(mp, MNT_FORCE, curthread);
+	if (error != 0) {
+		printf("unmount of %s failed (", mp->mnt_stat.f_mntonname);
+		if (error == EBUSY)
+			printf("BUSY)\n");
+		else
+			printf("%d)\n", error);
+	}
+}
+
 /*
  * Unmount all filesystems. The list is traversed in reverse order
  * of mounting to avoid dependencies.
@@ -3550,42 +3565,28 @@
 void
 vfs_unmountall(void)
 {
-	struct mount *mp;
-	struct thread *td;
-	int error;
+	struct mount *mp, *tmp;
 
 	CTR1(KTR_VFS, "%s: unmounting all filesystems", __func__);
-	td = curthread;
 
 	/*
 	 * Since this only runs when rebooting, it is not interlocked.
 	 */
-	while(!TAILQ_EMPTY(&mountlist)) {
-		mp = TAILQ_LAST(&mountlist, mntlist);
+	TAILQ_FOREACH_REVERSE_SAFE(mp, &mountlist, mntlist, mnt_list, tmp) {
 		vfs_ref(mp);
-		error = dounmount(mp, MNT_FORCE, td);
-		if (error != 0) {
-			TAILQ_REMOVE(&mountlist, mp, mnt_list);
-			/*
-			 * XXX: Due to the way in which we mount the root
-			 * file system off of devfs, devfs will generate a
-			 * "busy" warning when we try to unmount it before
-			 * the root.  Don't print a warning as a result in
-			 * order to avoid false positive errors that may
-			 * cause needless upset.
-			 */
-			if (strcmp(mp->mnt_vfc->vfc_name, "devfs") != 0) {
-				printf("unmount of %s failed (",
-				    mp->mnt_stat.f_mntonname);
-				if (error == EBUSY)
-					printf("BUSY)\n");
-				else
-					printf("%d)\n", error);
-			}
-		} else {
-			/* The unmount has removed mp from the mountlist */
-		}
+
+		/*
+		 * Forcibly unmounting "/dev" before "/" would prevent clean
+		 * unmount of the latter.
+		 */
+		if (mp == rootdevmp)
+			continue;
+
+		unmount_or_warn(mp);
 	}
+
+	if (rootdevmp != NULL)
+		unmount_or_warn(rootdevmp);
 }
 
 /*
Index: head/sys/sys/vnode.h
===================================================================
--- head/sys/sys/vnode.h
+++ head/sys/sys/vnode.h
@@ -420,6 +420,7 @@
  * Global vnode data.
  */
 extern	struct vnode *rootvnode;	/* root (i.e. "/") vnode */
+extern	struct mount *rootdevmp;	/* "/dev" mount */
 extern	int async_io_version;		/* 0 or POSIX version of AIO i'face */
 extern	int desiredvnodes;		/* number of vnodes desired */
 extern	struct uma_zone *namei_zone;