1 files changed, 21 insertions, 1 deletions

diff --git a/fs/btrfs/free-space-cache.c b/fs/btrfs/free-space-cache.c
index 4ba0aedc878b..74aa552f4793 100644
--- a/fs/btrfs/free-space-cache.c
+++ b/fs/btrfs/free-space-cache.c
@@ -75,7 +75,8 @@ static struct inode *__lookup_free_space_inode(struct btrfs_root *root,
          * sure NOFS is set to keep us from deadlocking.
          */
         nofs_flag = memalloc_nofs_save();
-        inode = btrfs_iget(fs_info->sb, &location, root, NULL);
+        inode = btrfs_iget_path(fs_info->sb, &location, root, NULL, path);
+        btrfs_release_path(path);
         memalloc_nofs_restore(nofs_flag);
         if (IS_ERR(inode))
                 return inode;
@@ -838,6 +839,25 @@ int load_free_space_cache(struct btrfs_fs_info *fs_info,
         path->search_commit_root = 1;
         path->skip_locking = 1;
 
+        /*
+         * We must pass a path with search_commit_root set to btrfs_iget in
+         * order to avoid a deadlock when allocating extents for the tree root.
+         *
+         * When we are COWing an extent buffer from the tree root, when looking
+         * for a free extent, at extent-tree.c:find_free_extent(), we can find
+         * block group without its free space cache loaded. When we find one
+         * we must load its space cache which requires reading its free space
+         * cache's inode item from the root tree. If this inode item is located
+         * in the same leaf that we started COWing before, then we end up in
+         * deadlock on the extent buffer (trying to read lock it when we
+         * previously write locked it).
+         *
+         * It's safe to read the inode item using the commit root because
+         * block groups, once loaded, stay in memory forever (until they are
+         * removed) as well as their space caches once loaded. New block groups
+         * once created get their ->cached field set to BTRFS_CACHE_FINISHED so
+         * we will never try to read their inode item while the fs is mounted.
+         */
         inode = lookup_free_space_inode(fs_info, block_group, path);
         if (IS_ERR(inode)) {
                 btrfs_free_path(path);