[PATCH net-next 3/3] net: rmnet: no longer rely on RTNL in rmnet_fill_info()

[Eric Dumazet <edumazet@google.com>]

rmnet_fill_info() used rmnet_get_port_rtnl(), and thus required
RTNL.

Now that priv->mux_id and port->data_format are properly
annotated, rmnet_fill_info() can simply fetch the port under
rcu_read_lock().

rmnet_get_port_rcu() was only used from the data path and thus
used rcu_dereference_bh(). Relax its lockdep condition so that it
can also be called from process context under a plain
rcu_read_lock(), and use it from rmnet_fill_info().

Note that rmnet_get_port_rcu() checks real_dev->rx_handler before
returning rx_handler_data: this is what makes the lockless lookup
safe against rmnet_unregister_real_device(), which clears
rx_handler, waits for a grace period in
netdev_rx_handler_unregister(), and only then frees the port.

While at it, add missing const qualifiers.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Subash Abhinov Kasiviswanathan <subash.a.kasiviswanathan@oss.qualcomm.com>
Cc: Sean Tranchetti <sean.tranchetti@oss.qualcomm.com>
---
 .../ethernet/qualcomm/rmnet/rmnet_config.c    | 38 +++++++++----------
 .../ethernet/qualcomm/rmnet/rmnet_config.h    |  2 +-
 2 files changed, 18 insertions(+), 22 deletions(-)

diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c b/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c
index 248a9d822409b7cbf3739c085487644b7025dd37..b930f638ec448de839806852a74fc4b2f8888e43 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c
@@ -371,32 +371,24 @@ static size_t rmnet_get_size(const struct net_device *dev)
 
 static int rmnet_fill_info(struct sk_buff *skb, const struct net_device *dev)
 {
-	struct rmnet_priv *priv = netdev_priv(dev);
-	struct net_device *real_dev;
+	const struct rmnet_priv *priv = netdev_priv(dev);
+	const struct rmnet_port *port;
 	struct ifla_rmnet_flags f;
-	struct rmnet_port *port;
 
-	real_dev = priv->real_dev;
+	if (nla_put_u16(skb, IFLA_RMNET_MUX_ID, READ_ONCE(priv->mux_id)))
+		return -EMSGSIZE;
 
-	if (nla_put_u16(skb, IFLA_RMNET_MUX_ID, priv->mux_id))
-		goto nla_put_failure;
-
-	if (rmnet_is_real_dev_registered(real_dev)) {
-		port = rmnet_get_port_rtnl(real_dev);
-		f.flags = port->data_format;
-	} else {
-		f.flags = 0;
-	}
+	rcu_read_lock();
+	port = rmnet_get_port_rcu(priv->real_dev);
+	f.flags = port ? READ_ONCE(port->data_format) : 0;
+	rcu_read_unlock();
 
 	f.mask  = ~0;
 
 	if (nla_put(skb, IFLA_RMNET_FLAGS, sizeof(f), &f))
-		goto nla_put_failure;
+		return -EMSGSIZE;
 
 	return 0;
-
-nla_put_failure:
-	return -EMSGSIZE;
 }
 
 struct rtnl_link_ops rmnet_link_ops __read_mostly = {
@@ -413,12 +405,16 @@ struct rtnl_link_ops rmnet_link_ops __read_mostly = {
 	.fill_info	= rmnet_fill_info,
 };
 
-struct rmnet_port *rmnet_get_port_rcu(struct net_device *real_dev)
+/* Can be called from a RCU read-side critical section, with or
+ * without BH disabled.
+ */
+struct rmnet_port *rmnet_get_port_rcu(const struct net_device *real_dev)
 {
-	if (rmnet_is_real_dev_registered(real_dev))
-		return rcu_dereference_bh(real_dev->rx_handler_data);
-	else
+	if (!rmnet_is_real_dev_registered(real_dev))
 		return NULL;
+
+	return rcu_dereference_check(real_dev->rx_handler_data,
+				     rcu_read_lock_bh_held());
 }
 
 struct rmnet_endpoint *rmnet_get_endpoint(struct rmnet_port *port, u8 mux_id)
diff --git a/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.h b/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.h
index f50fae1c6bdd5d4edfb18476e2ca44057921a472..5adda0323dda776d69d50d22bc75456b43442e3d 100644
--- a/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.h
+++ b/drivers/net/ethernet/qualcomm/rmnet/rmnet_config.h
@@ -90,7 +90,7 @@ struct rmnet_priv {
 	struct rmnet_priv_stats stats;
 };
 
-struct rmnet_port *rmnet_get_port_rcu(struct net_device *real_dev);
+struct rmnet_port *rmnet_get_port_rcu(const struct net_device *real_dev);
 struct rmnet_endpoint *rmnet_get_endpoint(struct rmnet_port *port, u8 mux_id);
 int rmnet_add_bridge(struct net_device *rmnet_dev,
 		     struct net_device *slave_dev,
-- 
2.55.0.1032.g73a4cd73de-goog