Btrfs: add a barrier before a waitqueue_active check

We were missing wakeups on the delayed ref waitqueue due
to races on waitqueue_active.

Signed-off-by: Chris Mason <chris.mason@fusionio.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 44f06201f376..4e1b153b7c47 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -5294,6 +5294,7 @@ static noinline int check_ref_cleanup(struct btrfs_trans_handle *trans,
         rb_erase(&head->node.rb_node, &delayed_refs->root);
 
         delayed_refs->num_entries--;
+        smp_mb();
         if (waitqueue_active(&root->fs_info->tree_mod_seq_wait))
                 wake_up(&root->fs_info->tree_mod_seq_wait);