From 6f7ff6d7832c6be13e8c95598884dbc40ad69fb7 Mon Sep 17 00:00:00 2001
From: Filipe Manana <fdmanana@suse.com>
Date: Wed, 2 Jul 2014 20:07:54 +0100
Subject: [PATCH] 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>
---
 fs/btrfs/backref.c | 3 +++
 1 file changed, 3 insertions(+)

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;