Btrfs: fix accessing the root pointer in tree mod log functions

The tree mod log functions were accessing root->node->... directly, without
use of btrfs_root_node() or explicit rcu locking. This could lead to an
extent buffer reference being leaked and another reference being freed too
early when preemtion was enabled.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/ctree.c

@@ -1069,11 +1069,11 @@ static noinline int __btrfs_cow_block(struct btrfs_trans_handle *trans,
  */
 static struct tree_mod_elem *
 __tree_mod_log_oldest_root(struct btrfs_fs_info *fs_info,
-			   struct btrfs_root *root, u64 time_seq)
+			   struct extent_buffer *eb_root, u64 time_seq)
 {
 	struct tree_mod_elem *tm;
 	struct tree_mod_elem *found = NULL;
-	u64 root_logical = root->node->start;
+	u64 root_logical = eb_root->start;
 	int looped = 0;
 
 	if (!time_seq)
@@ -1107,7 +1107,6 @@ __tree_mod_log_oldest_root(struct btrfs_fs_info *fs_info,
 
 		found = tm;
 		root_logical = tm->old_root.logical;
-		BUG_ON(root_logical == root->node->start);
 		looped = 1;
 	}
 
@@ -1245,30 +1244,31 @@ static inline struct extent_buffer *
 get_old_root(struct btrfs_root *root, u64 time_seq)
 {
 	struct tree_mod_elem *tm;
-	struct extent_buffer *eb;
+	struct extent_buffer *eb = NULL;
+	struct extent_buffer *eb_root;
 	struct extent_buffer *old;
 	struct tree_mod_root *old_root = NULL;
 	u64 old_generation = 0;
 	u64 logical;
 	u32 blocksize;
 
-	eb = btrfs_read_lock_root_node(root);
-	tm = __tree_mod_log_oldest_root(root->fs_info, root, time_seq);
+	eb_root = btrfs_read_lock_root_node(root);
+	tm = __tree_mod_log_oldest_root(root->fs_info, eb_root, time_seq);
 	if (!tm)
-		return root->node;
+		return eb_root;
 
 	if (tm->op == MOD_LOG_ROOT_REPLACE) {
 		old_root = &tm->old_root;
 		old_generation = tm->generation;
 		logical = old_root->logical;
 	} else {
-		logical = root->node->start;
+		logical = eb_root->start;
 	}
 
 	tm = tree_mod_log_search(root->fs_info, logical, time_seq);
 	if (old_root && tm && tm->op != MOD_LOG_KEY_REMOVE_WHILE_FREEING) {
-		btrfs_tree_read_unlock(root->node);
-		free_extent_buffer(root->node);
+		btrfs_tree_read_unlock(eb_root);
+		free_extent_buffer(eb_root);
 		blocksize = btrfs_level_size(root, old_root->level);
 		old = read_tree_block(root, logical, blocksize, 0);
 		if (!old) {
@@ -1280,13 +1280,13 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
 			free_extent_buffer(old);
 		}
 	} else if (old_root) {
-		btrfs_tree_read_unlock(root->node);
-		free_extent_buffer(root->node);
+		btrfs_tree_read_unlock(eb_root);
+		free_extent_buffer(eb_root);
 		eb = alloc_dummy_extent_buffer(logical, root->nodesize);
 	} else {
-		eb = btrfs_clone_extent_buffer(root->node);
-		btrfs_tree_read_unlock(root->node);
-		free_extent_buffer(root->node);
+		eb = btrfs_clone_extent_buffer(eb_root);
+		btrfs_tree_read_unlock(eb_root);
+		free_extent_buffer(eb_root);
 	}
 
 	if (!eb)
@@ -1296,7 +1296,7 @@ get_old_root(struct btrfs_root *root, u64 time_seq)
 	if (old_root) {
 		btrfs_set_header_bytenr(eb, eb->start);
 		btrfs_set_header_backref_rev(eb, BTRFS_MIXED_BACKREF_REV);
-		btrfs_set_header_owner(eb, root->root_key.objectid);
+		btrfs_set_header_owner(eb, btrfs_header_owner(eb_root));
 		btrfs_set_header_level(eb, old_root->level);
 		btrfs_set_header_generation(eb, old_generation);
 	}
@@ -1313,15 +1313,15 @@ int btrfs_old_root_level(struct btrfs_root *root, u64 time_seq)
 {
 	struct tree_mod_elem *tm;
 	int level;
+	struct extent_buffer *eb_root = btrfs_root_node(root);
 
-	tm = __tree_mod_log_oldest_root(root->fs_info, root, time_seq);
+	tm = __tree_mod_log_oldest_root(root->fs_info, eb_root, time_seq);
 	if (tm && tm->op == MOD_LOG_ROOT_REPLACE) {
 		level = tm->old_root.level;
 	} else {
-		rcu_read_lock();
-		level = btrfs_header_level(root->node);
-		rcu_read_unlock();
+		level = btrfs_header_level(eb_root);
 	}
+	free_extent_buffer(eb_root);
 
 	return level;
 }