Btrfs: fix possible deadlock when opening a seed device

The correct lock order is uuid_mutex -> volume_mutex -> chunk_mutex,
but when we mount a filesystem which has backing seed devices, we have
this lock chain:

    open_ctree()
        lock(chunk_mutex);
        read_chunk_tree();
            read_one_dev();
                open_seed_devices();
                    lock(uuid_mutex);

and then we hit a lockdep splat.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 563ef650850e..fbb493b28d5a 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -3506,7 +3506,7 @@ static int open_seed_devices(struct btrfs_root *root, u8 *fsid)
         struct btrfs_fs_devices *fs_devices;
         int ret;
 
-        mutex_lock(&uuid_mutex);
+        BUG_ON(!mutex_is_locked(&uuid_mutex));
 
         fs_devices = root->fs_info->fs_devices->seed;
         while (fs_devices) {
@@ -3544,7 +3544,6 @@ static int open_seed_devices(struct btrfs_root *root, u8 *fsid)
         fs_devices->seed = root->fs_info->fs_devices->seed;
         root->fs_info->fs_devices->seed = fs_devices;
 out:
-        mutex_unlock(&uuid_mutex);
         return ret;
 }
 
@@ -3687,6 +3686,9 @@ int btrfs_read_chunk_tree(struct btrfs_root *root)
         if (!path)
                 return -ENOMEM;
 
+        mutex_lock(&uuid_mutex);
+        lock_chunks(root);
+
         /* first we search for all of the device items, and then we
          * read in all of the chunk items.  This way we can create chunk
          * mappings that reference all of the devices that are afound
@@ -3737,6 +3739,9 @@ again:
         }
         ret = 0;
 error:
+        unlock_chunks(root);
+        mutex_unlock(&uuid_mutex);
+
         btrfs_free_path(path);
         return ret;
 }