Index: sys/kern/vfs_mountroot.c
===================================================================
--- sys/kern/vfs_mountroot.c
+++ sys/kern/vfs_mountroot.c
@@ -132,6 +132,11 @@
 static int root_mount_timeout = 3;
 TUNABLE_INT("vfs.mountroot.timeout", &root_mount_timeout);
 
+static int root_mount_always_wait = 0;
+SYSCTL_INT(_vfs, OID_AUTO, root_mount_always_wait, CTLFLAG_RDTUN,
+    &root_mount_always_wait, 0,
+    "Wait for root mount holds even if the root device already exists");
+
 SYSCTL_PROC(_vfs, OID_AUTO, root_mount_hold,
     CTLTYPE_STRING | CTLFLAG_RD | CTLFLAG_MPSAFE,
     NULL, 0, sysctl_vfs_root_mount_hold, "A",
@@ -961,10 +966,11 @@
 
 	/*
 	 * In case of ZFS and NFS we don't have a way to wait for
-	 * specific device.
+	 * specific device.  Also do the wait if the user forced that
+	 * behaviour by setting vfs.root_mount_hold_unconditional=1.
 	 */
 	if (strcmp(fs, "zfs") == 0 || strstr(fs, "nfs") != NULL ||
-	    dev[0] == '\0') {
+	    dev[0] == '\0' || root_mount_always_wait != 0) {
 		vfs_mountroot_wait();
 		return (0);
 	}