btrfs: hold a ref on the root in build_backref_tree

This is trickier than the previous conversions.  We have backref_node's
that need to hold onto their root for their lifetime.  Do the read of
the root and grab the ref.  If at any point we don't use the root we
discard it, however if we use it in our backref node we don't free it
until we free the backref node.  Any time we switch the root's for the
backref node we need to drop our ref on the old root and grab the ref on
the new root, and if we dupe a node we need to get a ref on the root
there as well.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -303,6 +303,7 @@ static void free_backref_node(struct backref_cache *cache,
 {
 	if (node) {
 		cache->nr_nodes--;
+		btrfs_put_fs_root(node->root);
 		kfree(node);
 	}
 }
@@ -636,7 +637,7 @@ static struct btrfs_root *find_reloc_root(struct reloc_control *rc,
 		root = (struct btrfs_root *)node->data;
 	}
 	spin_unlock(&rc->reloc_root_tree.lock);
-	return root;
+	return btrfs_grab_fs_root(root);
 }
 
 static int is_cowonly_root(u64 root_objectid)
@@ -938,6 +939,10 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			err = PTR_ERR(root);
 			goto out;
 		}
+		if (!btrfs_grab_fs_root(root)) {
+			err = -ENOENT;
+			goto out;
+		}
 
 		if (!test_bit(BTRFS_ROOT_REF_COWS, &root->state))
 			cur->cowonly = 1;
@@ -946,10 +951,12 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			/* tree root */
 			ASSERT(btrfs_root_bytenr(&root->root_item) ==
 			       cur->bytenr);
-			if (should_ignore_root(root))
+			if (should_ignore_root(root)) {
+				btrfs_put_fs_root(root);
 				list_add(&cur->list, &useless);
-			else
+			} else {
 				cur->root = root;
+			}
 			break;
 		}
 
@@ -962,6 +969,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		ret = btrfs_search_slot(NULL, root, node_key, path2, 0, 0);
 		path2->lowest_level = 0;
 		if (ret < 0) {
+			btrfs_put_fs_root(root);
 			err = ret;
 			goto out;
 		}
@@ -977,6 +985,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 				  root->root_key.objectid,
 				  node_key->objectid, node_key->type,
 				  node_key->offset);
+			btrfs_put_fs_root(root);
 			err = -ENOENT;
 			goto out;
 		}
@@ -988,15 +997,18 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			if (!path2->nodes[level]) {
 				ASSERT(btrfs_root_bytenr(&root->root_item) ==
 				       lower->bytenr);
-				if (should_ignore_root(root))
+				if (should_ignore_root(root)) {
+					btrfs_put_fs_root(root);
 					list_add(&lower->list, &useless);
-				else
+				} else {
 					lower->root = root;
+				}
 				break;
 			}
 
 			edge = alloc_backref_edge(cache);
 			if (!edge) {
+				btrfs_put_fs_root(root);
 				err = -ENOMEM;
 				goto out;
 			}
@@ -1006,6 +1018,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			if (!rb_node) {
 				upper = alloc_backref_node(cache);
 				if (!upper) {
+					btrfs_put_fs_root(root);
 					free_backref_edge(cache, edge);
 					err = -ENOMEM;
 					goto out;
@@ -1053,8 +1066,10 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			edge->node[LOWER] = lower;
 			edge->node[UPPER] = upper;
 
-			if (rb_node)
+			if (rb_node) {
+				btrfs_put_fs_root(root);
 				break;
+			}
 			lower = upper;
 			upper = NULL;
 		}
@@ -1291,7 +1306,8 @@ static int clone_backref_node(struct btrfs_trans_handle *trans,
 	new_node->level = node->level;
 	new_node->lowest = node->lowest;
 	new_node->checked = 1;
-	new_node->root = dest;
+	new_node->root = btrfs_grab_fs_root(dest);
+	ASSERT(new_node->root);
 
 	if (!node->lowest) {
 		list_for_each_entry(edge, &node->lower, list[UPPER]) {
@@ -2669,7 +2685,9 @@ struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
 			BUG_ON(next->new_bytenr);
 			BUG_ON(!list_empty(&next->list));
 			next->new_bytenr = root->node->start;
-			next->root = root;
+			btrfs_put_fs_root(next->root);
+			next->root = btrfs_grab_fs_root(root);
+			ASSERT(next->root);
 			list_add_tail(&next->list,
 				      &rc->backref_cache.changed);
 			__mark_block_processed(rc, next);
@@ -3141,7 +3159,9 @@ static int relocate_tree_block(struct btrfs_trans_handle *trans,
 			btrfs_record_root_in_trans(trans, root);
 			root = root->reloc_root;
 			node->new_bytenr = root->node->start;
-			node->root = root;
+			btrfs_put_fs_root(node->root);
+			node->root = btrfs_grab_fs_root(root);
+			ASSERT(node->root);
 			list_add_tail(&node->list, &rc->backref_cache.changed);
 		} else {
 			path->lowest_level = node->level;