!
! MFp4 bz_ipv6_fast:
!
!   Greatly reduce the scope of scope6 lock coverage where not needed.
!   Add comments to help figuring out caller context more easily.
!
!   Split in6_setscope() into an unlocked wrapper and a _locked()
!   version.  While this was mostly for analysing performance
!   difference from external callers it also turned out a nice split
!   adding the shortcut `not interested in any results in these cases'.
!
!   In in6_clearscope() do not do a write unless needed given we check
!   for the condition already.
!
!   Sponsored by:	The FreeBSD Foundation
!   Sponsored by:	iXsystems
!
! Reviewed by:		gnn (along with the big batch)
! MFC After:		6 weeks
!
--- sys/netinet6/scope6.c.orig	2012-04-25 07:34:21.000000000 +0000
+++ sys/netinet6/scope6.c	2012-05-02 12:00:20.000000000 +0000
@@ -114,6 +114,9 @@ scope6_ifdetach(struct scope6_id *sid)
 	free(sid, M_IFADDR);
 }
 
+/*
+ * Called from SIOCSSCOPE6.
+ */
 int
 scope6_set(struct ifnet *ifp, struct scope6_id *idlist)
 {
@@ -139,7 +142,6 @@ scope6_set(struct ifnet *ifp, struct sco
 	 * interface addresses, routing table entries, PCB entries...
 	 */
 
-	SCOPE6_LOCK();
 	for (i = 0; i < 16; i++) {
 		if (idlist->s6id_list[i] &&
 		    idlist->s6id_list[i] != sid->s6id_list[i]) {
@@ -150,7 +152,6 @@ scope6_set(struct ifnet *ifp, struct sco
 			if (i == IPV6_ADDR_SCOPE_INTFACELOCAL &&
 			    idlist->s6id_list[i] != ifp->if_index) {
 				IF_AFDATA_UNLOCK(ifp);
-				SCOPE6_UNLOCK();
 				return (EINVAL);
 			}
 
@@ -163,7 +164,6 @@ scope6_set(struct ifnet *ifp, struct sco
 				 * safety in later use.
 				 */
 				IF_AFDATA_UNLOCK(ifp);
-				SCOPE6_UNLOCK();
 				return (EINVAL);
 			}
 
@@ -175,7 +175,6 @@ scope6_set(struct ifnet *ifp, struct sco
 			sid->s6id_list[i] = idlist->s6id_list[i];
 		}
 	}
-	SCOPE6_UNLOCK();
 	IF_AFDATA_UNLOCK(ifp);
 
 	return (error);
@@ -193,9 +192,7 @@ scope6_get(struct ifnet *ifp, struct sco
 		return (EINVAL);
 	}
 
-	SCOPE6_LOCK();
 	*idlist = *sid;
-	SCOPE6_UNLOCK();
 
 	IF_AFDATA_UNLOCK(ifp);
 	return (0);
@@ -265,9 +262,9 @@ in6_addrscope(struct in6_addr *addr)
 }
 
 /*
- * ifp - note that this might be NULL
+ * Only called from nd6_setdefaultiface().
+ * ifp - note that this might be NULL.
  */
-
 void
 scope6_setdefault(struct ifnet *ifp)
 {
@@ -291,6 +288,9 @@ scope6_setdefault(struct ifnet *ifp)
 	SCOPE6_UNLOCK();
 }
 
+/*
+ * Only called for SIOCGSCOPE6DEF.
+ */
 int
 scope6_get_default(struct scope6_id *idlist)
 {
@@ -407,23 +407,15 @@ sa6_recoverscope(struct sockaddr_in6 *si
  *
  * ret_id - unnecessary?
  */
-int
-in6_setscope(struct in6_addr *in6, struct ifnet *ifp, u_int32_t *ret_id)
+static int
+in6_setscope_locked(struct in6_addr *in6, struct ifnet *ifp, u_int32_t *ret_id)
 {
 	int scope;
 	u_int32_t zoneid = 0;
 	struct scope6_id *sid;
 
-	IF_AFDATA_LOCK(ifp);
-
 	sid = SID(ifp);
-
-#ifdef DIAGNOSTIC
-	if (sid == NULL) { /* should not happen */
-		panic("in6_setscope: scope array is NULL");
-		/* NOTREACHED */
-	}
-#endif
+	KASSERT(sid != NULL, ("%s: ipf %p scope array is NULL", __func__, ifp));
 
 	/*
 	 * special case: the loopback address can only belong to a loopback
@@ -431,19 +423,16 @@ in6_setscope(struct in6_addr *in6, struc
 	 */
 	if (IN6_IS_ADDR_LOOPBACK(in6)) {
 		if (!(ifp->if_flags & IFF_LOOPBACK)) {
-			IF_AFDATA_UNLOCK(ifp);
 			return (EINVAL);
 		} else {
 			if (ret_id != NULL)
 				*ret_id = 0; /* there's no ambiguity */
-			IF_AFDATA_UNLOCK(ifp);
 			return (0);
 		}
 	}
 
 	scope = in6_addrscope(in6);
 
-	SCOPE6_LOCK();
 	switch (scope) {
 	case IPV6_ADDR_SCOPE_INTFACELOCAL: /* should be interface index */
 		zoneid = sid->s6id_list[IPV6_ADDR_SCOPE_INTFACELOCAL];
@@ -465,8 +454,6 @@ in6_setscope(struct in6_addr *in6, struc
 		zoneid = 0;	/* XXX: treat as global. */
 		break;
 	}
-	SCOPE6_UNLOCK();
-	IF_AFDATA_UNLOCK(ifp);
 
 	if (ret_id != NULL)
 		*ret_id = zoneid;
@@ -477,6 +464,26 @@ in6_setscope(struct in6_addr *in6, struc
 	return (0);
 }
 
+int
+in6_setscope(struct in6_addr *in6, struct ifnet *ifp, u_int32_t *ret_id)
+{
+	int error;
+
+	/*
+	 * In case we will not do anything, do an immediate return.  This avoids
+	 * taking the (rather expensive) lock.
+	 */
+	if (!IN6_IS_SCOPE_LINKLOCAL(in6) && !IN6_IS_ADDR_MC_INTFACELOCAL(in6) &&
+	    !IN6_IS_ADDR_LOOPBACK(in6) && ret_id == NULL)
+		return (0);
+
+	IF_AFDATA_RLOCK(ifp);
+	error = in6_setscope_locked(in6, ifp, ret_id);
+	IF_AFDATA_RUNLOCK(ifp);
+
+	return (error);
+}
+
 /*
  * Just clear the embedded scope identifier.  Return 0 if the original address
  * is intact; return non 0 if the address is modified.
@@ -487,9 +494,10 @@ in6_clearscope(struct in6_addr *in6)
 	int modified = 0;
 
 	if (IN6_IS_SCOPE_LINKLOCAL(in6) || IN6_IS_ADDR_MC_INTFACELOCAL(in6)) {
-		if (in6->s6_addr16[1] != 0)
+		if (in6->s6_addr16[1] != 0) {
 			modified = 1;
-		in6->s6_addr16[1] = 0;
+			in6->s6_addr16[1] = 0;
+		}
 	}
 
 	return (modified);