introduce reference counts for interfaces (ie, struct ifnet *ifp).

if_get can get a reference to an ifp, but it never releases that
reference. this provides an if_put function that can be used to
decrement the refcount.

we cannot come up with a scheme for letting the network stack run on
one (or many) cpus while ioctls are pulling interfaces down on another
cpu without refcounts for the interfaces.

if_put is going in now so we can go through the stack and put the
necessary calls to it in, and then we'll backfill this implementation
to actually check the refcounts when the interface detaches.

ok mpi@ mikeb@ claudio@

3 files changed, 27 insertions, 4 deletions

diff --git a/sys/net/if.c b/sys/net/if.c
index 83fc99725c5..fadedd86a96 100644
--- a/sys/net/if.c
+++ b/sys/net/if.c
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if.c,v 1.363 2015/09/01 04:56:55 dlg Exp $	*/
+/*	$OpenBSD: if.c,v 1.364 2015/09/09 16:01:10 dlg Exp $	*/
 /*	$NetBSD: if.c,v 1.35 1996/05/07 05:26:04 thorpej Exp $	*/
 
 /*
@@ -80,6 +80,7 @@
 #include <sys/domain.h>
 #include <sys/sysctl.h>
 #include <sys/task.h>
+#include <sys/atomic.h>
 
 #include <dev/rndvar.h>
 
@@ -260,7 +261,8 @@ if_attachsetup(struct ifnet *ifp)
 
 	if_addgroup(ifp, IFG_ALL);
 
-	ifindex2ifnet[if_index] = ifp;
+	ifp->if_refcnt = 0;
+	ifindex2ifnet[if_index] = if_ref(ifp);
 
 	if (ifp->if_snd.ifq_maxlen == 0)
 		IFQ_SET_MAXLEN(&ifp->if_snd, IFQ_MAXLEN);
@@ -664,6 +666,7 @@ if_detach(struct ifnet *ifp)
 	rt_ifannouncemsg(ifp, IFAN_DEPARTURE);
 
 	ifindex2ifnet[ifp->if_index] = NULL;
+	if_put(ifp);
 	splx(s);
 }
 
@@ -1231,9 +1234,26 @@ if_get(unsigned int index)
 	if (index < if_indexlim)
 		ifp = ifindex2ifnet[index];
 
+	return (if_ref(ifp));
+}
+
+struct ifnet *
+if_ref(struct ifnet *ifp)
+{
+	atomic_inc_int(&ifp->if_refcnt);
+
 	return (ifp);
 }
 
+void
+if_put(struct ifnet *ifp)
+{
+	if (ifp == NULL)
+		return;
+
+	atomic_dec_int(&ifp->if_refcnt);
+}
+
 /*
  * Interface ioctls.
  */
diff --git a/sys/net/if.h b/sys/net/if.h
index c4962b8df80..741ec2be537 100644
--- a/sys/net/if.h
+++ b/sys/net/if.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if.h,v 1.165 2015/08/30 10:39:16 mpi Exp $	*/
+/*	$OpenBSD: if.h,v 1.166 2015/09/09 16:01:10 dlg Exp $	*/
 /*	$NetBSD: if.h,v 1.23 1996/05/07 02:40:27 thorpej Exp $	*/
 
 /*
@@ -462,6 +462,8 @@ int	if_delgroup(struct ifnet *, const char *);
 void	if_group_routechange(struct sockaddr *, struct sockaddr *);
 struct	ifnet *ifunit(const char *);
 struct	ifnet *if_get(unsigned int);
+struct	ifnet *if_ref(struct ifnet *);
+void	if_put(struct ifnet *);
 void	ifnewlladdr(struct ifnet *);
 void	if_congestion(void);
 int	if_congested(void);
diff --git a/sys/net/if_var.h b/sys/net/if_var.h
index 034a64e301c..ba57aa11514 100644
--- a/sys/net/if_var.h
+++ b/sys/net/if_var.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: if_var.h,v 1.34 2015/07/02 09:40:02 mpi Exp $	*/
+/*	$OpenBSD: if_var.h,v 1.35 2015/09/09 16:01:10 dlg Exp $	*/
 /*	$NetBSD: if.h,v 1.23 1996/05/07 02:40:27 thorpej Exp $	*/
 
 /*
@@ -127,6 +127,7 @@ TAILQ_HEAD(ifnet_head, ifnet);		/* the actual queue head */
 
 struct ifnet {				/* and the entries */
 	void	*if_softc;		/* lower-level data for this if */
+	unsigned int if_refcnt;
 	TAILQ_ENTRY(ifnet) if_list;	/* all struct ifnets are chained */
 	TAILQ_ENTRY(ifnet) if_txlist;	/* list of ifnets ready to tx */
 	TAILQ_HEAD(, ifaddr) if_addrlist; /* linked list of addresses per if */