IPoIB: Fix deadlock on RTNL between bcast join comp and ipoib_stop()

Taking rtnl_lock in ipoib_mcast_join_complete() causes a deadlock with
ipoib_stop().  We avoid it by scheduling the piece of code that takes
the lock on ipoib_workqueue instead of executing it directly.  This
works because we only flush the ipoib_workqueue with the RTNL not held.

The deadlock happens because ipoib_stop() calls ipoib_ib_dev_down()
which calls ipoib_mcast_dev_flush(), which calls ipoib_mcast_free(),
which calls ipoib_mcast_leave(). The latter calls
ib_sa_free_multicast(), and this waits until the multicast completion
handler finishes.  This handler is ipoib_mcast_join_complete(), which
waits for the rtnl_lock(), which was already taken by ipoib_stop().

This bug was introduced in commit a77a57a1 ("IPoIB: Fix deadlock on
RTNL in ipoib_stop()").
Signed-off-by: Yossi Etigin <yosefe@voltaire.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/ipoib/ipoib.h

@@ -293,6 +293,7 @@ struct ipoib_dev_priv {
 
 	struct delayed_work pkey_poll_task;
 	struct delayed_work mcast_task;
+	struct work_struct carrier_on_task;
 	struct work_struct flush_light;
 	struct work_struct flush_normal;
 	struct work_struct flush_heavy;
@@ -464,6 +465,7 @@ int ipoib_dev_init(struct net_device *dev, struct ib_device *ca, int port);
 void ipoib_dev_cleanup(struct net_device *dev);
 
 void ipoib_mcast_join_task(struct work_struct *work);
+void ipoib_mcast_carrier_on_task(struct work_struct *work);
 void ipoib_mcast_send(struct net_device *dev, void *mgid, struct sk_buff *skb);
 
 void ipoib_mcast_restart_task(struct work_struct *work);

drivers/infiniband/ulp/ipoib/ipoib_main.c

@@ -1075,6 +1075,7 @@ static void ipoib_setup(struct net_device *dev)
 
 	INIT_DELAYED_WORK(&priv->pkey_poll_task, ipoib_pkey_poll);
 	INIT_DELAYED_WORK(&priv->mcast_task,   ipoib_mcast_join_task);
+	INIT_WORK(&priv->carrier_on_task, ipoib_mcast_carrier_on_task);
 	INIT_WORK(&priv->flush_light,   ipoib_ib_dev_flush_light);
 	INIT_WORK(&priv->flush_normal,   ipoib_ib_dev_flush_normal);
 	INIT_WORK(&priv->flush_heavy,   ipoib_ib_dev_flush_heavy);

drivers/infiniband/ulp/ipoib/ipoib_multicast.c

@@ -366,6 +366,21 @@ static int ipoib_mcast_sendonly_join(struct ipoib_mcast *mcast)
 	return ret;
 }
 
+void ipoib_mcast_carrier_on_task(struct work_struct *work)
+{
+	struct ipoib_dev_priv *priv = container_of(work, struct ipoib_dev_priv,
+						   carrier_on_task);
+
+	/*
+	 * Take rtnl_lock to avoid racing with ipoib_stop() and
+	 * turning the carrier back on while a device is being
+	 * removed.
+	 */
+	rtnl_lock();
+	netif_carrier_on(priv->dev);
+	rtnl_unlock();
+}
+
 static int ipoib_mcast_join_complete(int status,
 				     struct ib_sa_multicast *multicast)
 {
@@ -392,16 +407,12 @@ static int ipoib_mcast_join_complete(int status,
 					   &priv->mcast_task, 0);
 		mutex_unlock(&mcast_mutex);
 
-		if (mcast == priv->broadcast) {
 		/*
-			 * Take RTNL lock here to avoid racing with
-			 * ipoib_stop() and turning the carrier back
-			 * on while a device is being removed.
+		 * Defer carrier on work to ipoib_workqueue to avoid a
+		 * deadlock on rtnl_lock here.
 		 */
-			rtnl_lock();
-			netif_carrier_on(dev);
-			rtnl_unlock();
-		}
+		if (mcast == priv->broadcast)
+			queue_work(ipoib_workqueue, &priv->carrier_on_task);
 
 		return 0;
 	}