Btrfs: read lock extent buffer while walking backrefs

Before processing the extent buffer, acquire a read lock on it, so
that we're safe against concurrent updates on the extent buffer.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/fs/btrfs/backref.c b/fs/btrfs/backref.c
index e25564bfcb46..a1efd39ca28a 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -1001,8 +1001,11 @@ again:
                                         ret = -EIO;
                                         goto out;
                                 }
+                                btrfs_tree_read_lock(eb);
+                                btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
                                 ret = find_extent_in_eb(eb, bytenr,
                                                         *extent_item_pos, &eie);
+                                btrfs_tree_read_unlock_blocking(eb);
                                 free_extent_buffer(eb);
                                 if (ret < 0)
                                         goto out;