Index: head/sys/fs/nfs/nfs_var.h
===================================================================
--- head/sys/fs/nfs/nfs_var.h
+++ head/sys/fs/nfs/nfs_var.h
@@ -502,8 +502,8 @@
     int *, int *, int);
 int nfscl_getstateid(vnode_t, u_int8_t *, int, u_int32_t, int, struct ucred *,
     NFSPROC_T *, nfsv4stateid_t *, void **);
-void nfscl_ownerrelease(struct nfsclowner *, int, int, int);
-void nfscl_openrelease(struct nfsclopen *, int, int);
+void nfscl_ownerrelease(struct nfsmount *, struct nfsclowner *, int, int, int);
+void nfscl_openrelease(struct nfsmount *, struct nfsclopen *, int, int);
 int nfscl_getcl(struct mount *, struct ucred *, NFSPROC_T *, int,
     struct nfsclclient **);
 struct nfsclclient *nfscl_findcl(struct nfsmount *);
Index: head/sys/fs/nfs/nfsport.h
===================================================================
--- head/sys/fs/nfs/nfsport.h
+++ head/sys/fs/nfs/nfsport.h
@@ -929,6 +929,8 @@
 #define	NFSHASNOLAYOUTCOMMIT(n)	((n)->nm_state & NFSSTA_NOLAYOUTCOMMIT)
 #define	NFSHASSESSPERSIST(n)	((n)->nm_state & NFSSTA_SESSPERSIST)
 #define	NFSHASPNFS(n)		((n)->nm_state & NFSSTA_PNFS)
+#define	NFSHASONEOPENOWN(n)	(((n)->nm_flag & NFSMNT_ONEOPENOWN) != 0 &&	\
+				    (n)->nm_minorvers > 0)
 
 /*
  * Gets the stats field out of the mount structure.
Index: head/sys/fs/nfsclient/nfs_clport.c
===================================================================
--- head/sys/fs/nfsclient/nfs_clport.c
+++ head/sys/fs/nfsclient/nfs_clport.c
@@ -634,7 +634,7 @@
 	struct proc *p;
 
 	if (id == NULL) {
-		printf("NULL id\n");
+		/* Return the single open_owner of all 0 bytes. */
 		bzero(cp, NFSV4CL_LOCKNAMELEN);
 		return;
 	}
@@ -1255,7 +1255,14 @@
 	} tl;
 	struct proc *p;
 	pid_t pid;
-	int ret = 0;
+	int i, ret = 0;
+
+	/* For the single open_owner of all 0 bytes, just return 0. */
+	for (i = 0; i < NFSV4CL_LOCKNAMELEN; i++)
+		if (own[i] != 0)
+			break;
+	if (i == NFSV4CL_LOCKNAMELEN)
+		return (0);
 
 	tl.cval[0] = *own++;
 	tl.cval[1] = *own++;
Index: head/sys/fs/nfsclient/nfs_clrpcops.c
===================================================================
--- head/sys/fs/nfsclient/nfs_clrpcops.c
+++ head/sys/fs/nfsclient/nfs_clrpcops.c
@@ -347,7 +347,7 @@
 	     */
 	    if (!error)
 		op->nfso_opencnt++;
-	    nfscl_openrelease(op, error, newone);
+	    nfscl_openrelease(nmp, op, error, newone);
 	    if (error == NFSERR_GRACE || error == NFSERR_STALECLIENTID ||
 		error == NFSERR_STALEDONTRECOVER || error == NFSERR_DELAY ||
 		error == NFSERR_BADSESSION) {
@@ -1893,7 +1893,7 @@
 		if (dp != NULL)
 			(void) nfscl_deleg(nmp->nm_mountp, owp->nfsow_clp,
 			    (*nfhpp)->nfh_fh, (*nfhpp)->nfh_len, cred, p, &dp);
-		nfscl_ownerrelease(owp, error, newone, unlocked);
+		nfscl_ownerrelease(nmp, owp, error, newone, unlocked);
 		if (error == NFSERR_GRACE || error == NFSERR_STALECLIENTID ||
 		    error == NFSERR_STALEDONTRECOVER || error == NFSERR_DELAY ||
 		    error == NFSERR_BADSESSION) {
@@ -2198,7 +2198,7 @@
 				error = ret;
 		    }
 		}
-		nfscl_openrelease(op, error, newone);
+		nfscl_openrelease(nmp, op, error, newone);
 		*unlockedp = 1;
 	}
 	if (nd->nd_repstat != 0 && error == 0)
Index: head/sys/fs/nfsclient/nfs_clstate.c
===================================================================
--- head/sys/fs/nfsclient/nfs_clstate.c
+++ head/sys/fs/nfsclient/nfs_clstate.c
@@ -247,7 +247,6 @@
 	 * If none found, add the new one or return error, depending upon
 	 * "create".
 	 */
-	nfscl_filllockowner(p->td_proc, own, F_POSIX);
 	NFSLOCKCLSTATE();
 	dp = NULL;
 	/* First check the delegation list */
@@ -264,10 +263,17 @@
 		}
 	}
 
-	if (dp != NULL)
+	if (dp != NULL) {
+		nfscl_filllockowner(p->td_proc, own, F_POSIX);
 		ohp = &dp->nfsdl_owner;
-	else
+	} else {
+		/* For NFSv4.1 and this option, use a single open_owner. */
+		if (NFSHASONEOPENOWN(VFSTONFS(vnode_mount(vp))))
+			nfscl_filllockowner(NULL, own, F_POSIX);
+		else
+			nfscl_filllockowner(p->td_proc, own, F_POSIX);
 		ohp = &clp->nfsc_owner;
+	}
 	/* Now, search for an openowner */
 	LIST_FOREACH(owp, ohp, nfsow_list) {
 		if (!NFSBCMP(owp->nfsow_owner, own, NFSV4CL_LOCKNAMELEN))
@@ -300,9 +306,24 @@
 	/*
 	 * Serialize modifications to the open owner for multiple threads
 	 * within the same process using a read/write sleep lock.
+	 * For NFSv4.1 and a single OpenOwner, allow concurrent open operations
+	 * by acquiring a shared lock.  The close operations still use an
+	 * exclusive lock for this case.
 	 */
-	if (lockit)
-		nfscl_lockexcl(&owp->nfsow_rwlock, NFSCLSTATEMUTEXPTR);
+	if (lockit != 0) {
+		if (NFSHASONEOPENOWN(VFSTONFS(vnode_mount(vp)))) {
+			/*
+			 * Get a shared lock on the OpenOwner, but first
+			 * wait for any pending exclusive lock, so that the
+			 * exclusive locker gets priority.
+			 */
+			nfsv4_lock(&owp->nfsow_rwlock, 0, NULL,
+			    NFSCLSTATEMUTEXPTR, NULL);
+			nfsv4_getref(&owp->nfsow_rwlock, NULL,
+			    NFSCLSTATEMUTEXPTR, NULL);
+		} else
+			nfscl_lockexcl(&owp->nfsow_rwlock, NFSCLSTATEMUTEXPTR);
+	}
 	NFSUNLOCKCLSTATE();
 	if (nowp != NULL)
 		FREE((caddr_t)nowp, M_NFSCLOWNER);
@@ -545,7 +566,10 @@
 		 * If p != NULL, we want to search the parentage tree
 		 * for a matching OpenOwner and use that.
 		 */
-		nfscl_filllockowner(p->td_proc, own, F_POSIX);
+		if (NFSHASONEOPENOWN(VFSTONFS(vnode_mount(vp))))
+			nfscl_filllockowner(NULL, own, F_POSIX);
+		else
+			nfscl_filllockowner(p->td_proc, own, F_POSIX);
 		lp = NULL;
 		error = nfscl_getopen(&clp->nfsc_owner, nfhp, fhlen, own, own,
 		    mode, &lp, &op);
@@ -679,15 +703,19 @@
  * with the open owner.
  */
 APPLESTATIC void
-nfscl_ownerrelease(struct nfsclowner *owp, __unused int error,
-    __unused int candelete, int unlocked)
+nfscl_ownerrelease(struct nfsmount *nmp, struct nfsclowner *owp,
+    __unused int error, __unused int candelete, int unlocked)
 {
 
 	if (owp == NULL)
 		return;
 	NFSLOCKCLSTATE();
-	if (!unlocked)
-		nfscl_lockunlock(&owp->nfsow_rwlock);
+	if (unlocked == 0) {
+		if (NFSHASONEOPENOWN(nmp))
+			nfsv4_relref(&owp->nfsow_rwlock);
+		else
+			nfscl_lockunlock(&owp->nfsow_rwlock);
+	}
 	nfscl_clrelease(owp->nfsow_clp);
 	NFSUNLOCKCLSTATE();
 }
@@ -696,7 +724,8 @@
  * Release use of an open structure under an open owner.
  */
 APPLESTATIC void
-nfscl_openrelease(struct nfsclopen *op, int error, int candelete)
+nfscl_openrelease(struct nfsmount *nmp, struct nfsclopen *op, int error,
+    int candelete)
 {
 	struct nfsclclient *clp;
 	struct nfsclowner *owp;
@@ -705,7 +734,10 @@
 		return;
 	NFSLOCKCLSTATE();
 	owp = op->nfso_own;
-	nfscl_lockunlock(&owp->nfsow_rwlock);
+	if (NFSHASONEOPENOWN(nmp))
+		nfsv4_relref(&owp->nfsow_rwlock);
+	else
+		nfscl_lockunlock(&owp->nfsow_rwlock);
 	clp = owp->nfsow_clp;
 	if (error && candelete && op->nfso_opencnt == 0)
 		nfscl_freeopen(op, 0);
@@ -997,7 +1029,10 @@
 	} else {
 		nfscl_filllockowner(id, own, flags);
 		ownp = own;
-		nfscl_filllockowner(p->td_proc, openown, F_POSIX);
+		if (NFSHASONEOPENOWN(VFSTONFS(vnode_mount(vp))))
+			nfscl_filllockowner(NULL, openown, F_POSIX);
+		else
+			nfscl_filllockowner(p->td_proc, openown, F_POSIX);
 		openownp = openown;
 	}
 	if (!recovery) {
@@ -1725,6 +1760,7 @@
 	struct nfsclowner *owp, *nowp;
 	struct nfsclopen *op;
 	struct nfscllockowner *lp, *nlp;
+	struct nfscldeleg *dp;
 
 	NFSPROCLISTLOCK();
 	NFSLOCKCLSTATE();
@@ -1738,6 +1774,20 @@
 		if (nfscl_procdoesntexist(owp->nfsow_owner))
 			nfscl_cleanup_common(clp, owp->nfsow_owner);
 	}
+
+	/*
+	 * For the single open_owner case, these lock owners need to be
+	 * checked to see if they still exist separately.
+	 * This is because nfscl_procdoesntexist() never returns true for
+	 * the single open_owner so that the above doesn't ever call
+	 * nfscl_cleanup_common().
+	 */
+	TAILQ_FOREACH(dp, &clp->nfsc_deleg, nfsdl_list) {
+		LIST_FOREACH_SAFE(lp, &dp->nfsdl_lock, nfsl_list, nlp) {
+			if (nfscl_procdoesntexist(lp->nfsl_owner))
+				nfscl_cleanup_common(clp, lp->nfsl_owner);
+		}
+	}
 	NFSUNLOCKCLSTATE();
 	NFSPROCLISTUNLOCK();
 }
Index: head/sys/fs/nfsclient/nfs_clvfsops.c
===================================================================
--- head/sys/fs/nfsclient/nfs_clvfsops.c
+++ head/sys/fs/nfsclient/nfs_clvfsops.c
@@ -592,6 +592,12 @@
 		nmp->nm_flag &= ~NFSMNT_RDIRPLUS;
 	}
 
+	/* Clear ONEOPENOWN for NFSv2, 3 and 4.0. */
+	if (nmp->nm_minorvers == 0) {
+		argp->flags &= ~NFSMNT_ONEOPENOWN;
+		nmp->nm_flag &= ~NFSMNT_ONEOPENOWN;
+	}
+
 	/* Re-bind if rsrvd port requested and wasn't on one */
 	adjsock = !(nmp->nm_flag & NFSMNT_RESVPORT)
 		  && (argp->flags & NFSMNT_RESVPORT);
@@ -727,7 +733,7 @@
     "resvport", "readahead", "hostname", "timeo", "timeout", "addr", "fh",
     "nfsv3", "sec", "principal", "nfsv4", "gssname", "allgssname", "dirpath",
     "minorversion", "nametimeo", "negnametimeo", "nocto", "noncontigwr",
-    "pnfs", "wcommitsize",
+    "pnfs", "wcommitsize", "oneopenown",
     NULL };
 
 /*
@@ -962,6 +968,8 @@
 		args.flags |= NFSMNT_NONCONTIGWR;
 	if (vfs_getopt(mp->mnt_optnew, "pnfs", NULL, NULL) == 0)
 		args.flags |= NFSMNT_PNFS;
+	if (vfs_getopt(mp->mnt_optnew, "oneopenown", NULL, NULL) == 0)
+		args.flags |= NFSMNT_ONEOPENOWN;
 	if (vfs_getopt(mp->mnt_optnew, "readdirsize", (void **)&opt, NULL) == 0) {
 		if (opt == NULL) { 
 			vfs_mount_error(mp, "illegal readdirsize");
@@ -1172,8 +1180,8 @@
 
 		/*
 		 * When doing an update, we can't change version,
-		 * security, switch lockd strategies or change cookie
-		 * translation
+		 * security, switch lockd strategies, change cookie
+		 * translation or switch oneopenown.
 		 */
 		args.flags = (args.flags &
 		    ~(NFSMNT_NFSV3 |
@@ -1181,6 +1189,7 @@
 		      NFSMNT_KERB |
 		      NFSMNT_INTEGRITY |
 		      NFSMNT_PRIVACY |
+		      NFSMNT_ONEOPENOWN |
 		      NFSMNT_NOLOCKD /*|NFSMNT_XLATECOOKIE*/)) |
 		    (nmp->nm_flag &
 			(NFSMNT_NFSV3 |
@@ -1188,6 +1197,7 @@
 			 NFSMNT_KERB |
 			 NFSMNT_INTEGRITY |
 			 NFSMNT_PRIVACY |
+			 NFSMNT_ONEOPENOWN |
 			 NFSMNT_NOLOCKD /*|NFSMNT_XLATECOOKIE*/));
 		nfs_decode_args(mp, nmp, &args, NULL, td->td_ucred, td);
 		goto out;
@@ -1946,6 +1956,8 @@
 		    &blen);
 		nfscl_printopt(nmp, (nmp->nm_flag & NFSMNT_PNFS) != 0, ",pnfs",
 		    &buf, &blen);
+		nfscl_printopt(nmp, (nmp->nm_flag & NFSMNT_ONEOPENOWN) != 0 &&
+		    nmp->nm_minorvers > 0, ",oneopenown", &buf, &blen);
 	}
 	nfscl_printopt(nmp, (nmp->nm_flag & NFSMNT_NFSV3) != 0, "nfsv3", &buf,
 	    &blen);
Index: head/sys/nfsclient/nfsargs.h
===================================================================
--- head/sys/nfsclient/nfsargs.h
+++ head/sys/nfsclient/nfsargs.h
@@ -76,7 +76,7 @@
 #define	NFSMNT_MAXGRPS		0x00000020  /* set maximum grouplist size */
 #define	NFSMNT_INT		0x00000040  /* allow interrupts on hard mount */
 #define	NFSMNT_NOCONN		0x00000080  /* Don't Connect the socket */
-/* 0x100 free, was NFSMNT_NQNFS */
+#define	NFSMNT_ONEOPENOWN	0x00000100  /* Use one OpenOwner for NFSv4.1 */
 #define	NFSMNT_NFSV3		0x00000200  /* Use NFS Version 3 protocol */
 #define	NFSMNT_KERB		0x00000400  /* Use RPCSEC_GSS/Krb5 */
 #define	NFSMNT_DUMBTIMR		0x00000800  /* Don't estimate rtt dynamically */