[PATCH] Btrfs: fix deadlock with freeze and sync

[PATCH] Btrfs: fix deadlock with freeze and sync

[Josef Bacik]

We can deadlock with freeze right now because we unconditionally start a
transaction in our ->sync_fs() call.  To fix this just check and see if we
have a running transaction to commit.  This saves us from the deadlock
because at this point we'll have the umount sem for the sb so we're safe
from freezes coming in after we've done our check.  With this patch the
freeze xfstests no longer deadlocks.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
---
 fs/btrfs/super.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 7b65fe8..950accc 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -849,11 +849,17 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
 
 	btrfs_wait_ordered_extents(root, 0, 0);
 
-	trans = btrfs_start_transaction(root, 0);
+	spin_lock(&fs_info->trans_lock);
+	if (!fs_info->running_transaction) {
+		spin_unlock(&fs_info->trans_lock);
+		return 0;
+	}
+	spin_unlock(&fs_info->trans_lock);
+
+	trans = btrfs_join_transaction(root);
 	if (IS_ERR(trans))
 		return PTR_ERR(trans);
-	ret = btrfs_commit_transaction(trans, root);
-	return ret;
+	return btrfs_commit_transaction(trans, root);
 }
 
 static int btrfs_show_options(struct seq_file *seq, struct dentry *dentry)
-- 
1.7.7.6