Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -117,6 +117,21 @@
 static void	vnlru_return_batches(struct vfsops *mnt_op);
 static void	destroy_vpollinfo(struct vpollinfo *vi);
 
+/*
+ * These fences are intended for cases where some synchronization is
+ * needed around lockless vnode refcount updates.  Access to vnode fields
+ * is generally synchronized by the interlock, but we have some assertions
+ * which check vnode flags without acquiring the lock.  Thus, these
+ * fences are INVARIANTS-only for now.
+ */
+#ifdef INVARIANTS
+#define	VNODE_REFCOUNT_FENCE_ACQ()	atomic_thread_fence_acq()
+#define	VNODE_REFCOUNT_FENCE_REL()	atomic_thread_fence_rel()
+#else
+#define	VNODE_REFCOUNT_FENCE_ACQ()
+#define	VNODE_REFCOUNT_FENCE_REL()
+#endif
+
 /*
  * Number of vnodes in existence.  Increased whenever getnewvnode()
  * allocates a new vnode, decreased in vdropl() for VI_DOOMED vnode.
@@ -1018,6 +1033,7 @@
 		 */
 		freevnodes--;
 		vp->v_iflag &= ~VI_FREE;
+		VNODE_REFCOUNT_FENCE_REL();
 		refcount_acquire(&vp->v_holdcnt);
 
 		mtx_unlock(&vnode_free_list_mtx);
@@ -2495,6 +2511,7 @@
 
 	if (vp->v_type != VCHR &&
 	    refcount_acquire_if_not_zero(&vp->v_usecount)) {
+		VNODE_REFCOUNT_FENCE_ACQ();
 		VNASSERT((vp->v_iflag & VI_OWEINACT) == 0, vp,
 		    ("vnode with usecount and VI_OWEINACT set"));
 	} else {
@@ -2593,6 +2610,7 @@
 		} else {
 			oweinact = 1;
 			vp->v_iflag &= ~VI_OWEINACT;
+			VNODE_REFCOUNT_FENCE_REL();
 		}
 		refcount_acquire(&vp->v_usecount);
 		v_incr_devcount(vp);
@@ -2807,6 +2825,7 @@
 	CTR2(KTR_VFS, "%s: vp %p", __func__, vp);
 	if (!locked) {
 		if (refcount_acquire_if_not_zero(&vp->v_holdcnt)) {
+			VNODE_REFCOUNT_FENCE_ACQ();
 			VNASSERT((vp->v_iflag & VI_FREE) == 0, vp,
 			    ("_vhold: vnode with holdcnt is free"));
 			return;