mctp: Add refcounts to mctp_dev

Currently, we tie the struct mctp_dev lifetime to the underlying struct
net_device, and hold/put that device as a proxy for a separate mctp_dev
refcount. This works because we're not holding any references to the
mctp_dev that are different from the netdev lifetime.

In a future change we'll break that assumption though, as we'll need to
hold mctp_dev references in a workqueue, which might live past the
netdev unregister notification.

In order to support that, this change introduces a refcount on the
mctp_dev, currently taken by the net_device->mctp_ptr reference, and
released on netdev unregister events. We can then use this for future
references that might outlast the net device.
Signed-off-by: Jeremy Kerr <jk@codeconstruct.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/mctpdevice.h

@@ -17,6 +17,8 @@
 struct mctp_dev {
 	struct net_device	*dev;
 
+	refcount_t		refs;
+
 	unsigned int		net;
 
 	/* Only modified under RTNL. Reads have addrs_lock held */
@@ -32,4 +34,7 @@ struct mctp_dev {
 struct mctp_dev *mctp_dev_get_rtnl(const struct net_device *dev);
 struct mctp_dev *__mctp_dev_get(const struct net_device *dev);
 
+void mctp_dev_hold(struct mctp_dev *mdev);
+void mctp_dev_put(struct mctp_dev *mdev);
+
 #endif /* __NET_MCTPDEVICE_H */

net/mctp/device.c

@@ -35,14 +35,6 @@ struct mctp_dev *mctp_dev_get_rtnl(const struct net_device *dev)
 	return rtnl_dereference(dev->mctp_ptr);
 }
 
-static void mctp_dev_destroy(struct mctp_dev *mdev)
-{
-	struct net_device *dev = mdev->dev;
-
-	dev_put(dev);
-	kfree_rcu(mdev, rcu);
-}
-
 static int mctp_fill_addrinfo(struct sk_buff *skb, struct netlink_callback *cb,
 			      struct mctp_dev *mdev, mctp_eid_t eid)
 {
@@ -255,6 +247,19 @@ static int mctp_rtm_deladdr(struct sk_buff *skb, struct nlmsghdr *nlh,
 	return 0;
 }
 
+void mctp_dev_hold(struct mctp_dev *mdev)
+{
+	refcount_inc(&mdev->refs);
+}
+
+void mctp_dev_put(struct mctp_dev *mdev)
+{
+	if (refcount_dec_and_test(&mdev->refs)) {
+		dev_put(mdev->dev);
+		kfree_rcu(mdev, rcu);
+	}
+}
+
 static struct mctp_dev *mctp_add_dev(struct net_device *dev)
 {
 	struct mctp_dev *mdev;
@@ -270,7 +275,9 @@ static struct mctp_dev *mctp_add_dev(struct net_device *dev)
 	mdev->net = mctp_default_net(dev_net(dev));
 
 	/* associate to net_device */
+	refcount_set(&mdev->refs, 1);
 	rcu_assign_pointer(dev->mctp_ptr, mdev);
+
 	dev_hold(dev);
 	mdev->dev = dev;
 
@@ -345,7 +352,7 @@ static void mctp_unregister(struct net_device *dev)
 	mctp_neigh_remove_dev(mdev);
 	kfree(mdev->addrs);
 
-	mctp_dev_destroy(mdev);
+	mctp_dev_put(mdev);
 }
 
 static int mctp_register(struct net_device *dev)

net/mctp/neigh.c

@@ -47,7 +47,7 @@ static int mctp_neigh_add(struct mctp_dev *mdev, mctp_eid_t eid,
 	}
 	INIT_LIST_HEAD(&neigh->list);
 	neigh->dev = mdev;
-	dev_hold(neigh->dev->dev);
+	mctp_dev_hold(neigh->dev);
 	neigh->eid = eid;
 	neigh->source = source;
 	memcpy(neigh->ha, lladdr, lladdr_len);
@@ -63,7 +63,7 @@ static void __mctp_neigh_free(struct rcu_head *rcu)
 {
 	struct mctp_neigh *neigh = container_of(rcu, struct mctp_neigh, rcu);
 
-	dev_put(neigh->dev->dev);
+	mctp_dev_put(neigh->dev);
 	kfree(neigh);
 }
 

net/mctp/route.c

@@ -455,7 +455,7 @@ static int mctp_route_output(struct mctp_route *route, struct sk_buff *skb)
 static void mctp_route_release(struct mctp_route *rt)
 {
 	if (refcount_dec_and_test(&rt->refs)) {
-		dev_put(rt->dev->dev);
+		mctp_dev_put(rt->dev);
 		kfree_rcu(rt, rcu);
 	}
 }
@@ -815,7 +815,7 @@ static int mctp_route_add(struct mctp_dev *mdev, mctp_eid_t daddr_start,
 	rt->max = daddr_start + daddr_extent;
 	rt->mtu = mtu;
 	rt->dev = mdev;
-	dev_hold(rt->dev->dev);
+	mctp_dev_hold(rt->dev);
 	rt->type = type;
 	rt->output = rtfn;