btrfs: reloc: remove the open-coded goto loop for breadth-first search

build_backref_tree() uses "goto again;" to implement a breadth-first
search to build backref cache.

This patch will extract most of its work into a wrapper,
handle_one_tree_block(), and use a do {} while() loop to implement the
same thing.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/relocation.c

@@ -957,77 +957,31 @@ static int handle_indirect_tree_backref(struct backref_cache *cache,
 	return ret;
 }
 
-/*
- * build backref tree for a given tree block. root of the backref tree
- * corresponds the tree block, leaves of the backref tree correspond
- * roots of b-trees that reference the tree block.
- *
- * the basic idea of this function is check backrefs of a given block
- * to find upper level blocks that reference the block, and then check
- * backrefs of these upper level blocks recursively. the recursion stop
- * when tree root is reached or backrefs for the block is cached.
- *
- * NOTE: if we find backrefs for a block are cached, we know backrefs
- * for all upper level blocks that directly/indirectly reference the
- * block are also cached.
- */
-static noinline_for_stack
-struct backref_node *build_backref_tree(struct reloc_control *rc,
+static int handle_one_tree_block(struct backref_cache *cache,
+				 struct btrfs_path *path,
+				 struct btrfs_backref_iter *iter,
 				 struct btrfs_key *node_key,
-					int level, u64 bytenr)
+				 struct backref_node *cur)
 {
-	struct btrfs_backref_iter *iter;
-	struct backref_cache *cache = &rc->backref_cache;
-	/* For searching parent of TREE_BLOCK_REF */
-	struct btrfs_path *path;
-	struct backref_node *cur;
-	struct backref_node *upper;
-	struct backref_node *lower;
-	struct backref_node *node = NULL;
-	struct backref_node *exist = NULL;
+	struct btrfs_fs_info *fs_info = cache->fs_info;
 	struct backref_edge *edge;
-	struct rb_node *rb_node;
-	int cowonly;
+	struct backref_node *exist;
 	int ret;
-	int err = 0;
-
-	iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
-	if (!iter)
-		return ERR_PTR(-ENOMEM);
-	path = btrfs_alloc_path();
-	if (!path) {
-		err = -ENOMEM;
-		goto out;
-	}
 
-	node = alloc_backref_node(cache, bytenr, level);
-	if (!node) {
-		err = -ENOMEM;
-		goto out;
-	}
-
-	node->lowest = 1;
-	cur = node;
-again:
 	ret = btrfs_backref_iter_start(iter, cur->bytenr);
-	if (ret < 0) {
-		err = ret;
-		goto out;
-	}
-
+	if (ret < 0)
+		return ret;
 	/*
 	 * We skip the first btrfs_tree_block_info, as we don't use the key
 	 * stored in it, but fetch it from the tree block
 	 */
 	if (btrfs_backref_has_tree_block_info(iter)) {
 		ret = btrfs_backref_iter_next(iter);
-		if (ret < 0) {
-			err = ret;
+		if (ret < 0)
 			goto out;
-		}
 		/* No extra backref? This means the tree block is corrupted */
 		if (ret > 0) {
-			err = -EUCLEAN;
+			ret = -EUCLEAN;
 			goto out;
 		}
 	}
@@ -1070,7 +1024,7 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 			type = btrfs_get_extent_inline_ref_type(eb, iref,
 							BTRFS_REF_TYPE_BLOCK);
 			if (type == BTRFS_REF_TYPE_INVALID) {
-				err = -EUCLEAN;
+				ret = -EUCLEAN;
 				goto out;
 			}
 			key.type = type;
@@ -1096,16 +1050,13 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		/* SHARED_BLOCK_REF means key.offset is the parent bytenr */
 		if (key.type == BTRFS_SHARED_BLOCK_REF_KEY) {
 			ret = handle_direct_tree_backref(cache, &key, cur);
-			if (ret < 0) {
-				err = ret;
+			if (ret < 0)
 				goto out;
-			}
 			continue;
 		} else if (unlikely(key.type == BTRFS_EXTENT_REF_V0_KEY)) {
-			err = -EINVAL;
-			btrfs_print_v0_err(rc->extent_root->fs_info);
-			btrfs_handle_fs_error(rc->extent_root->fs_info, err,
-					      NULL);
+			ret = -EINVAL;
+			btrfs_print_v0_err(fs_info);
+			btrfs_handle_fs_error(fs_info, ret, NULL);
 			goto out;
 		} else if (key.type != BTRFS_TREE_BLOCK_REF_KEY) {
 			continue;
@@ -1118,29 +1069,85 @@ struct backref_node *build_backref_tree(struct reloc_control *rc,
 		 */
 		ret = handle_indirect_tree_backref(cache, path, &key, node_key,
 						   cur);
-		if (ret < 0) {
-			err = ret;
+		if (ret < 0)
+			goto out;
+	}
+	ret = 0;
+	cur->checked = 1;
+	WARN_ON(exist);
+out:
+	btrfs_backref_iter_release(iter);
+	return ret;
+}
+
+/*
+ * Build backref tree for a given tree block. Root of the backref tree
+ * corresponds the tree block, leaves of the backref tree correspond roots of
+ * b-trees that reference the tree block.
+ *
+ * The basic idea of this function is check backrefs of a given block to find
+ * upper level blocks that reference the block, and then check backrefs of
+ * these upper level blocks recursively. The recursion stops when tree root is
+ * reached or backrefs for the block is cached.
+ *
+ * NOTE: if we find that backrefs for a block are cached, we know backrefs for
+ * all upper level blocks that directly/indirectly reference the block are also
+ * cached.
+ */
+static noinline_for_stack struct backref_node *build_backref_tree(
+			struct reloc_control *rc, struct btrfs_key *node_key,
+			int level, u64 bytenr)
+{
+	struct btrfs_backref_iter *iter;
+	struct backref_cache *cache = &rc->backref_cache;
+	/* For searching parent of TREE_BLOCK_REF */
+	struct btrfs_path *path;
+	struct backref_node *cur;
+	struct backref_node *upper;
+	struct backref_node *lower;
+	struct backref_node *node = NULL;
+	struct backref_edge *edge;
+	struct rb_node *rb_node;
+	int cowonly;
+	int ret;
+	int err = 0;
+
+	iter = btrfs_backref_iter_alloc(rc->extent_root->fs_info, GFP_NOFS);
+	if (!iter)
+		return ERR_PTR(-ENOMEM);
+	path = btrfs_alloc_path();
+	if (!path) {
+		err = -ENOMEM;
 		goto out;
 	}
+
+	node = alloc_backref_node(cache, bytenr, level);
+	if (!node) {
+		err = -ENOMEM;
+		goto out;
 	}
+
+	node->lowest = 1;
+	cur = node;
+
+	/* Breadth-first search to build backref cache */
+	do {
+		ret = handle_one_tree_block(cache, path, iter, node_key, cur);
 		if (ret < 0) {
 			err = ret;
 			goto out;
 		}
-	ret = 0;
-	btrfs_backref_iter_release(iter);
-
-	cur->checked = 1;
-	WARN_ON(exist);
-
-	/* the pending list isn't empty, take the first block to process */
-	if (!list_empty(&cache->pending_edge)) {
-		edge = list_first_entry(&cache->pending_edge,
+		edge = list_first_entry_or_null(&cache->pending_edge,
 				struct backref_edge, list[UPPER]);
+		/*
+		 * The pending list isn't empty, take the first block to
+		 * process
+		 */
+		if (edge) {
 			list_del_init(&edge->list[UPPER]);
 			cur = edge->node[UPPER];
-		goto again;
 		}
+	} while (edge);
 
 	/*
 	 * everything goes well, connect backref nodes and insert backref nodes