Btrfs: fix barrier flushes

When btrfs is writing the super blocks, it send barrier flushes to make
sure writeback caching drives get all the metadata on disk in the
right order.

But, we have two bugs in the way these are sent down.  When doing
full commits (not via the tree log), we are sending the barrier down
before the last super when it should be going down before the first.

In multi-device setups, we should be waiting for the barriers to
complete on all devices before writing any of the supers.

Both of these bugs can cause corruptions on power failures.  We fix it
with some new code to send down empty barriers to all devices before
writing the first super.

Alexandre Oliva found the multi-device bug.  Arne Jansen did the async
barrier loop.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
Reported-by: Alexandre Oliva <oliva@lsd.ic.unicamp.br>

1 files changed, 6 insertions, 0 deletions

diff --git a/fs/btrfs/volumes.h b/fs/btrfs/volumes.h
index ab5b1c49f352..78f2d4d4f37f 100644
--- a/fs/btrfs/volumes.h
+++ b/fs/btrfs/volumes.h
@@ -100,6 +100,12 @@ struct btrfs_device {
         struct reada_zone *reada_curr_zone;
         struct radix_tree_root reada_zones;
         struct radix_tree_root reada_extents;
+
+        /* for sending down flush barriers */
+        struct bio *flush_bio;
+        struct completion flush_wait;
+        int nobarriers;
+
 };
 
 struct btrfs_fs_devices {