Index: sys/cddl/compat/opensolaris/kern/opensolaris_vfs.c
===================================================================
--- sys/cddl/compat/opensolaris/kern/opensolaris_vfs.c
+++ sys/cddl/compat/opensolaris/kern/opensolaris_vfs.c
@@ -242,6 +242,7 @@
 	if (VFS_ROOT(mp, LK_EXCLUSIVE, &mvp))
 		panic("mount: lost mount");
 	VOP_UNLOCK(vp, 0);
+	vfs_fastpath_enable_mp(mp);
 	vfs_unbusy(mp);
 	*vpp = mvp;
 	return (0);
Index: sys/kern/vfs_default.c
===================================================================
--- sys/kern/vfs_default.c
+++ sys/kern/vfs_default.c
@@ -51,6 +51,7 @@
 #include <sys/malloc.h>
 #include <sys/mount.h>
 #include <sys/namei.h>
+#include <sys/rmlock.h>
 #include <sys/rwlock.h>
 #include <sys/fcntl.h>
 #include <sys/unistd.h>
@@ -586,6 +587,7 @@
 		struct mount **a_mpp;
 	} */ *ap;
 {
+	struct rm_priotracker tracker;
 	struct mount *mp;
 	struct vnode *vp;
 
@@ -598,6 +600,19 @@
 	 */
 	vp = ap->a_vp;
 	mp = vp->v_mount;
+	if (mp == NULL)
+		goto out;
+	if (vfs_fastpath_enter_mp(mp, &tracker)) {
+		if (mp != vp->v_mount) {
+			vfs_fastpath_exit_mp(mp, &tracker);
+			mp = NULL;
+			goto out;
+		}
+		MNT_REF_FASTPATH(mp);
+		vfs_fastpath_exit_mp(mp, &tracker);
+		goto out;
+	}
+
 	MNT_ILOCK(mp);
 	if (mp != vp->v_mount) {
 		MNT_IUNLOCK(mp);
Index: sys/kern/vfs_mount.c
===================================================================
--- sys/kern/vfs_mount.c
+++ sys/kern/vfs_mount.c
@@ -58,6 +58,7 @@
 #include <sys/sbuf.h>
 #include <sys/syscallsubr.h>
 #include <sys/sysproto.h>
+#include <sys/rmlock.h>
 #include <sys/sx.h>
 #include <sys/sysctl.h>
 #include <sys/sysent.h>
@@ -123,6 +124,9 @@
 	mtx_init(&mp->mnt_mtx, "struct mount mtx", NULL, MTX_DEF);
 	mtx_init(&mp->mnt_listmtx, "struct mount vlist mtx", NULL, MTX_DEF);
 	lockinit(&mp->mnt_explock, PVFS, "explock", 0, 0);
+	rm_init(&mp->mnt_rmlock, "struct mount rmlock");
+	mp->mnt_ref = 0;
+	mp->mnt_fastpathdisabled = 1;
 	return (0);
 }
 
@@ -132,6 +136,7 @@
 	struct mount *mp;
 
 	mp = (struct mount *)mem;
+	rm_destroy(&mp->mnt_rmlock);
 	lockdestroy(&mp->mnt_explock);
 	mtx_destroy(&mp->mnt_listmtx);
 	mtx_destroy(&mp->mnt_mtx);
@@ -443,8 +448,15 @@
 void
 vfs_ref(struct mount *mp)
 {
+	struct rm_priotracker tracker;
 
 	CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
+	if (vfs_fastpath_enter_mp(mp, &tracker)) {
+		MNT_REF_FASTPATH(mp);
+		vfs_fastpath_exit_mp(mp, &tracker);
+		return;
+	}
+
 	MNT_ILOCK(mp);
 	MNT_REF(mp);
 	MNT_IUNLOCK(mp);
@@ -453,8 +465,15 @@
 void
 vfs_rel(struct mount *mp)
 {
+	struct rm_priotracker tracker;
 
 	CTR2(KTR_VFS, "%s: mp %p", __func__, mp);
+	if (vfs_fastpath_enter_mp(mp, &tracker)) {
+		MNT_REL_FASTPATH(mp);
+		vfs_fastpath_exit_mp(mp, &tracker);
+		return;
+	}
+
 	MNT_ILOCK(mp);
 	MNT_REL(mp);
 	MNT_IUNLOCK(mp);
@@ -478,7 +497,12 @@
 	mp->mnt_activevnodelistsize = 0;
 	TAILQ_INIT(&mp->mnt_tmpfreevnodelist);
 	mp->mnt_tmpfreevnodelistsize = 0;
-	mp->mnt_ref = 0;
+	if (mp->mnt_ref != 0 || mp->mnt_lockref != 0 ||
+	    mp->mnt_writeopcount != 0)
+		panic("%s: non-zero counters on new mp %p\n", __func__, mp);
+	if (mp->mnt_fastpathdisabled != 1)
+		panic("%s:fastpathdisabled should be 1 but %d found\n", __func__,
+		    mp->mnt_fastpathdisabled);
 	(void) vfs_busy(mp, MBF_NOWAIT);
 	atomic_add_acq_int(&vfsp->vfc_refcount, 1);
 	mp->mnt_op = vfsp->vfc_vfsops;
@@ -507,6 +531,9 @@
 vfs_mount_destroy(struct mount *mp)
 {
 
+	if (mp->mnt_fastpathdisabled == 0)
+		panic("%s: entered with fast path enabled\n", __func__);
+
 	MNT_ILOCK(mp);
 	mp->mnt_kern_flag |= MNTK_REFEXPIRE;
 	if (mp->mnt_kern_flag & MNTK_MWAIT) {
@@ -540,6 +567,11 @@
 	if (mp->mnt_lockref != 0)
 		panic("vfs_mount_destroy: nonzero lock refcount");
 	MNT_IUNLOCK(mp);
+
+	if (mp->mnt_fastpathdisabled != 1)
+		panic("%s:fastpathdisabled should be 1 but %d found\n", __func__,
+		    mp->mnt_fastpathdisabled);
+
 	if (mp->mnt_vnodecovered != NULL)
 		vrele(mp->mnt_vnodecovered);
 #ifdef MAC
@@ -951,6 +983,7 @@
 	vrele(newdp);
 	if ((mp->mnt_flag & MNT_RDONLY) == 0)
 		vfs_allocate_syncvnode(mp);
+	vfs_fastpath_enable_mp(mp);
 	vfs_unbusy(mp);
 	return (0);
 }
@@ -1019,6 +1052,8 @@
 	VI_UNLOCK(vp);
 	VOP_UNLOCK(vp, 0);
 
+	vfs_fastpath_disable_mp(mp);
+
 	MNT_ILOCK(mp);
 	if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0) {
 		MNT_IUNLOCK(mp);
@@ -1100,6 +1135,7 @@
 	else
 		vfs_deallocate_syncvnode(mp);
 end:
+	vfs_fastpath_enable_mp(mp);
 	vfs_unbusy(mp);
 	VI_LOCK(vp);
 	vp->v_iflag &= ~VI_MOUNT;
@@ -1336,6 +1372,29 @@
 	vn_finished_write(mp);
 }
 
+void
+vfs_fastpath_disable_mp(struct mount *mp)
+{
+
+	rm_wlock(&mp->mnt_rmlock);
+	MNT_ILOCK(mp);
+	mp->mnt_fastpathdisabled++;
+	MNT_IUNLOCK(mp);
+	rm_wunlock(&mp->mnt_rmlock);
+}
+
+void
+vfs_fastpath_enable_mp(struct mount *mp)
+{
+
+	MNT_ILOCK(mp);
+	if (mp->mnt_fastpathdisabled <= 0)
+		panic("%s: invalid fastpathdisabled count %d for mp %p\n",
+		    __func__, mp->mnt_fastpathdisabled, mp);
+	mp->mnt_fastpathdisabled--;
+	MNT_IUNLOCK(mp);
+}
+
 /*
  * Do the actual filesystem unmount.
  */
@@ -1379,13 +1438,16 @@
 		return (error);
 	}
 
+	vfs_fastpath_disable_mp(mp);
+
 	vn_start_write(NULL, &mp, V_WAIT | V_MNTREF);
 	MNT_ILOCK(mp);
 	if ((mp->mnt_kern_flag & MNTK_UNMOUNT) != 0 ||
 	    (mp->mnt_flag & MNT_UPDATE) != 0 ||
 	    !TAILQ_EMPTY(&mp->mnt_uppers)) {
 		dounmount_cleanup(mp, coveredvp, 0);
-		return (EBUSY);
+		error = EBUSY;
+		goto out_error;
 	}
 	mp->mnt_kern_flag |= MNTK_UNMOUNT;
 	if (flags & MNT_NONBUSY) {
@@ -1394,7 +1456,7 @@
 		MNT_ILOCK(mp);
 		if (error != 0) {
 			dounmount_cleanup(mp, coveredvp, MNTK_UNMOUNT);
-			return (error);
+			goto out_error;
 		}
 	}
 	/* Allow filesystems to detect that a forced unmount is in progress. */
@@ -1472,7 +1534,7 @@
 		MNT_IUNLOCK(mp);
 		if (coveredvp)
 			VOP_UNLOCK(coveredvp, 0);
-		return (error);
+		goto out_error;
 	}
 	mtx_lock(&mountlist_mtx);
 	TAILQ_REMOVE(&mountlist, mp, mnt_list);
@@ -1491,6 +1553,9 @@
 		rootdevmp = NULL;
 	vfs_mount_destroy(mp);
 	return (0);
+out_error:
+	vfs_fastpath_enable_mp(mp);
+	return (error);
 }
 
 /*
Index: sys/kern/vfs_mountroot.c
===================================================================
--- sys/kern/vfs_mountroot.c
+++ sys/kern/vfs_mountroot.c
@@ -273,6 +273,7 @@
 
 		*mpp = mp;
 		rootdevmp = mp;
+		vfs_fastpath_enable_mp(mp);
 	}
 
 	set_rootvnode();
Index: sys/kern/vfs_subr.c
===================================================================
--- sys/kern/vfs_subr.c
+++ sys/kern/vfs_subr.c
@@ -3904,6 +3904,7 @@
 	    mp->mnt_secondary_accwrites);
 	db_printf("    mnt_gjprovider = %s\n",
 	    mp->mnt_gjprovider != NULL ? mp->mnt_gjprovider : "NULL");
+	db_printf("    mnt_fastpathdisabled = %d\n", mp->mnt_fastpathdisabled);
 
 	db_printf("\n\nList of active vnodes\n");
 	TAILQ_FOREACH(vp, &mp->mnt_activevnodelist, v_actfreelist) {
Index: sys/sys/mount.h
===================================================================
--- sys/sys/mount.h
+++ sys/sys/mount.h
@@ -41,8 +41,10 @@
 #include <sys/lock.h>
 #include <sys/lockmgr.h>
 #include <sys/tslog.h>
+#include <sys/_cpuset.h>
 #include <sys/_mutex.h>
 #include <sys/_sx.h>
+#include <sys/_rmlock.h>
 #endif
 
 /*
@@ -226,6 +228,8 @@
 	struct lock	mnt_explock;		/* vfs_export walkers lock */
 	TAILQ_ENTRY(mount) mnt_upper_link;	/* (m) we in the all uppers */
 	TAILQ_HEAD(, mount) mnt_uppers;		/* (m) upper mounts over us*/
+	struct rmlock	mnt_rmlock;
+	int		mnt_fastpathdisabled;	/* (i) is fast path disabled */
 };
 
 /*
@@ -265,15 +269,26 @@
 #define	MNT_ITRYLOCK(mp) mtx_trylock(&(mp)->mnt_mtx)
 #define	MNT_IUNLOCK(mp)	mtx_unlock(&(mp)->mnt_mtx)
 #define	MNT_MTX(mp)	(&(mp)->mnt_mtx)
+
+#define	MNT_REF_FASTPATH(mp)	do {					\
+	atomic_add_int(&(mp)->mnt_ref, 1);				\
+} while (0)
+#define	MNT_REL_FASTPATH(mp)	do {					\
+	int _c;								\
+	_c = atomic_fetchadd_int(&(mp)->mnt_ref, -1) - 1;		\
+	KASSERT(_c >= 0, ("negative mnt_ref %d", _c));			\
+} while (0)
+
 #define	MNT_REF(mp)	do {						\
 	mtx_assert(MNT_MTX(mp), MA_OWNED);				\
-	(mp)->mnt_ref++;						\
+	atomic_add_int(&(mp)->mnt_ref, 1);				\
 } while (0)
 #define	MNT_REL(mp)	do {						\
+	int _c;								\
 	mtx_assert(MNT_MTX(mp), MA_OWNED);				\
-	KASSERT((mp)->mnt_ref > 0, ("negative mnt_ref"));		\
-	(mp)->mnt_ref--;						\
-	if ((mp)->mnt_ref == 0)						\
+	_c = atomic_fetchadd_int(&(mp)->mnt_ref, -1) - 1;		\
+	KASSERT(_c >= 0, ("negative mnt_ref %d", _c));			\
+	if (_c == 0)							\
 		wakeup((mp));						\
 } while (0)
 
@@ -929,6 +944,24 @@
 void	syncer_suspend(void);
 void	syncer_resume(void);
 
+void	vfs_fastpath_disable_mp(struct mount *);
+void	vfs_fastpath_enable_mp(struct mount *);
+
+#define vfs_fastpath_enter_mp(mp, tracker) ({			\
+	struct mount *_mp = (mp);				\
+	int _retval;						\
+	_retval = rm_try_rlock(&(_mp)->mnt_rmlock, tracker);	\
+	if (__predict_true(_retval != 0)) {			\
+		if (__predict_false(_mp->mnt_fastpathdisabled)) {\
+			vfs_fastpath_exit_mp(_mp, tracker);	\
+			_retval = 0;				\
+		}						\
+	}							\
+	_retval;						\
+})
+
+#define vfs_fastpath_exit_mp(mp, tracker) rm_runlock(&(mp)->mnt_rmlock, tracker)
+
 #else /* !_KERNEL */
 
 #include <sys/cdefs.h>