IPoIB: Fix multicast race between canceling and completing

ipoib_mcast_stop_thread currently tests mcast->query and if it is
NULL, does not perform wait_for_completion on the mcast and frees the
mcast object directly.

However, since both operations are done without locking, it is
possible that ipoib_mcast_join_complete is in progress on this mcast
object and has set mcast->query to NULL already.

Solve this by:
- taking priv->lock before we change mcast->query in ipoib_mcast_join_complete,
  and keeping it until we no longer need the mcast object
- taking priv->lock around mcast->query test in ipoib_mcast_stop_thread

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/ulp/ipoib/ipoib_multicast.c

@@ -437,9 +437,11 @@ static void ipoib_mcast_join_complete(int status,
 	if (mcast->backoff > IPOIB_MAX_BACKOFF_SECONDS)
 		mcast->backoff = IPOIB_MAX_BACKOFF_SECONDS;
 
+	mutex_lock(&mcast_mutex);
+
+	spin_lock_irq(&priv->lock);
 	mcast->query = NULL;
 
-	mutex_lock(&mcast_mutex);
 	if (test_bit(IPOIB_MCAST_RUN, &priv->flags)) {
 		if (status == -ETIMEDOUT)
 			queue_work(ipoib_workqueue, &priv->mcast_task);
@@ -448,6 +450,7 @@ static void ipoib_mcast_join_complete(int status,
 					   mcast->backoff * HZ);
 	} else
 		complete(&mcast->done);
+	spin_unlock_irq(&priv->lock);
 	mutex_unlock(&mcast_mutex);
 
 	return;
@@ -635,21 +638,27 @@ int ipoib_mcast_stop_thread(struct net_device *dev, int flush)
 	if (flush)
 		flush_workqueue(ipoib_workqueue);
 
+	spin_lock_irq(&priv->lock);
 	if (priv->broadcast && priv->broadcast->query) {
 		ib_sa_cancel_query(priv->broadcast->query_id, priv->broadcast->query);
 		priv->broadcast->query = NULL;
+		spin_unlock_irq(&priv->lock);
 		ipoib_dbg_mcast(priv, "waiting for bcast\n");
 		wait_for_completion(&priv->broadcast->done);
-	}
+	} else
+		spin_unlock_irq(&priv->lock);
 
 	list_for_each_entry(mcast, &priv->multicast_list, list) {
+		spin_lock_irq(&priv->lock);
 		if (mcast->query) {
 			ib_sa_cancel_query(mcast->query_id, mcast->query);
 			mcast->query = NULL;
+			spin_unlock_irq(&priv->lock);
 			ipoib_dbg_mcast(priv, "waiting for MGID " IPOIB_GID_FMT "\n",
 					IPOIB_GID_ARG(mcast->mcmember.mgid));
 			wait_for_completion(&mcast->done);
-		}
+		} else
+			spin_unlock_irq(&priv->lock);
 	}
 
 	return 0;