Btrfs: fix deadlock with freeze and sync V2

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

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 2e06f124f28494502c981380de1aa0ba4c144fdf..073c2368f45974651b98a78888028ba6078317ef 100644 (file)
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -813,7 +813,6 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
        struct btrfs_trans_handle *trans;
        struct btrfs_fs_info *fs_info = btrfs_sb(sb);
        struct btrfs_root *root = fs_info->tree_root;
-       int ret;
 
        trace_btrfs_sync_fs(wait);
 
@@ -824,11 +823,17 @@ 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)