Index: share/man/man9/if_get.9
===================================================================
RCS file: /cvs/src/share/man/man9/if_get.9,v
retrieving revision 1.2
diff -u -p -r1.2 if_get.9
--- share/man/man9/if_get.9	8 Dec 2015 18:35:51 -0000	1.2
+++ share/man/man9/if_get.9	30 Mar 2016 12:52:25 -0000
@@ -19,7 +19,9 @@
 .Os
 .Sh NAME
 .Nm if_get ,
-.Nm if_put
+.Nm if_put ,
+.Nm if_enter ,
+.Nm if_leave
 .Nd get an interface pointer from an interface index
 .Sh SYNOPSIS
 .In net/if.h
@@ -27,36 +29,89 @@
 .Fn if_get "unsigned int ifidx"
 .Ft void
 .Fn if_put "struct ifnet *ifp"
+.Ft struct ifnet *
+.Fn if_enter "struct ifref *ref" "unsigned int ifidx"
+.Ft void
+.Fn if_leave "struct ifref *ref" "struct ifnet *ifp"
 .Sh DESCRIPTION
-The
+The kernel maintains a unique identifier for each network interface
+present in the system called the interface index.
+This interface index can be used as an indirect reference to a
+network interface, and can then be translated into a reference to
+the actual ifnet structure as needed.
+.Pp
+If an interface is removed from the system, subsequent lookups using
+it's interface index will no longer return a valid pointer to an
+an ifnet structure.
+.Pp
+The index value
+.Dv 0
+is never associated with an interface descriptor and can be used
+to determine if an interface index is valid or not.
+This supports the use of the interface index as a scope identifier
+for IPv6 link local addresses, where scope 0 means no scope has
+been specified.
+It also supports the use of the interface index as the unique
+identifier for network interfaces in SNMP applications as per RFC
+2863.
+.Pp
 .Fn if_get
-function returns a pointer to the interface descriptor corresponding to the
+returns a pointer to the interface descriptor corresponding to the
 unique index
 .Fa ifidx .
 This descriptor is guaranteed to be valid until
 .Fn if_put
 is called on the returned pointer.
 .Pp
-The index value
-.Dv 0
-is never associated with an interface descriptor and can be used to determine if
-an interface index is valid or not.
-.Pp
-The
 .Fn if_put
-function releases a reference on the interface descriptor pointed by
+releases a reference on the interface descriptor pointed by
 .Fa ifp .
 If
 .Fa ifp
 is a
 .Dv NULL
 pointer, no action occurs.
+.Pp
+.Fn if_enter
+returns a pointer to the interface descriptor corresponding to the unique index
+.Fa ifidx .
+The reference is stored in the ifref structure
+.Fa ref
+provided by the caller.
+The descriptor is guaranteed to be valid until
+.Fn if_leave
+is called.
+.Pp
+.Fn if_leave
+releases the reference to the interface descriptor
+.Fa ifp
+which was stored in
+.Fa ref .
+.Pp
+All calls to
+.Fn if_enter
+must have a corresponding call to
+.Fn if_leave ,
+even if the pointer to the interface descriptor is
+.Dv NULL .
 .Sh CONTEXT
-.Fn if_get ,
+.Fn if_get
 and
 .Fn if_put
 can be called during autoconf, from process context, or from interrupt context.
+.Pp
+.Fn if_enter
+and
+.Fn if_leave
+can be called during autoconf, from process context, or from interrupt context.
+Calling
+.Fn if_leave
+from a different context or on a different CPU to the preceding
+.Fn if_enter
+call will lead to undefined behaviour.
 .Sh RETURN VALUES
 .Fn if_get
-returns a pointer to an interface descriptor if the index is valid, otherwise
+and
+.Fn if_enter
+return a pointer to an interface descriptor if the index is valid, otherwise
 .Dv NULL .
Index: sys/net/if.c
===================================================================
RCS file: /cvs/src/sys/net/if.c,v
retrieving revision 1.429
diff -u -p -r1.429 if.c
--- sys/net/if.c	16 Mar 2016 12:08:09 -0000	1.429
+++ sys/net/if.c	30 Mar 2016 12:52:25 -0000
@@ -770,6 +770,7 @@ if_input_process(void *xmq)
 	struct mbuf *m;
 	struct ifnet *ifp;
 	struct ifih *ifih;
+	struct ifref ref;
 	struct srpl_iter i;
 	int s;
 
@@ -781,10 +782,10 @@ if_input_process(void *xmq)
 
 	s = splnet();
 	while ((m = ml_dequeue(&ml)) != NULL) {
-		ifp = if_get(m->m_pkthdr.ph_ifidx);
+		ifp = if_enter(&ref, m->m_pkthdr.ph_ifidx);
 		if (ifp == NULL) {
 			m_freem(m);
-			continue;
+			goto next;
 		}
 
 		/*
@@ -800,7 +801,8 @@ if_input_process(void *xmq)
 		if (ifih == NULL)
 			m_freem(m);
 
-		if_put(ifp);
+next:
+		if_leave(&ref, ifp);
 	}
 	splx(s);
 }
@@ -1482,7 +1484,7 @@ ifunit(const char *name)
  * Map interface index to interface structure pointer.
  */
 struct ifnet *
-if_get(unsigned int index)
+if_enter(struct ifref *ref, unsigned int index)
 {
 	struct if_map *if_map;
 	struct srp *map;
@@ -1492,19 +1494,33 @@ if_get(unsigned int index)
 	if (index < if_map->limit) {
 		map = (struct srp *)(if_map + 1);
 
-		ifp = srp_follow(&if_idxmap.map, if_map, &map[index]);
-		if (ifp != NULL) {
-			KASSERT(ifp->if_index == index);
-			if_ref(ifp);
-		}
-		srp_leave(&map[index], ifp);
-	} else
+		ref->srp = &map[index];
+		ifp = srp_follow(&if_idxmap.map, if_map, ref->srp);
+	} else {
+		ref->srp = NULL;
 		srp_leave(&if_idxmap.map, if_map);
+	}
 
 	return (ifp);
 }
 
 struct ifnet *
+if_get(unsigned int index)
+{
+	struct ifref ref;
+	struct ifnet *ifp;
+
+	ifp = if_enter(&ref, index);
+	if (ifp != NULL) {
+		KASSERT(ifp->if_index == index);
+		if_ref(ifp);
+	}
+	if_leave(&ref, ifp);
+ 
+	return (ifp);
+}
+
+struct ifnet *
 if_ref(struct ifnet *ifp)
 {
 	refcnt_take(&ifp->if_refcnt);
@@ -1519,6 +1535,15 @@ if_put(struct ifnet *ifp)
 		return;
 
 	refcnt_rele_wake(&ifp->if_refcnt);
+}
+
+void
+if_leave(struct ifref *ref, struct ifnet *ifp)
+{
+	if (ref->srp == NULL)
+		return;
+
+	srp_leave(ref->srp, ifp);
 }
 
 int
Index: sys/net/if.h
===================================================================
RCS file: /cvs/src/sys/net/if.h,v
retrieving revision 1.176
diff -u -p -r1.176 if.h
--- sys/net/if.h	2 Mar 2016 00:00:16 -0000	1.176
+++ sys/net/if.h	30 Mar 2016 12:52:25 -0000
@@ -455,6 +455,11 @@ struct if_parent {
 #ifdef _KERNEL
 struct socket;
 struct ifnet;
+struct srp;
+
+struct ifref {
+	struct srp *srp;
+};
 
 void	if_alloc_sadl(struct ifnet *);
 void	if_free_sadl(struct ifnet *);
@@ -477,6 +482,8 @@ int	if_addgroup(struct ifnet *, const ch
 int	if_delgroup(struct ifnet *, const char *);
 void	if_group_routechange(struct sockaddr *, struct sockaddr *);
 struct	ifnet *ifunit(const char *);
+struct	ifnet *if_enter(struct ifref *, unsigned int);
+void	if_leave(struct ifref *, struct ifnet *);
 struct	ifnet *if_get(unsigned int);
 void	if_put(struct ifnet *);
 void	ifnewlladdr(struct ifnet *);