enforce ->sync_fs is only called for rw superblock

Make sure a superblock really is writeable by checking MS_RDONLY
under s_umount.  sync_filesystems needed some re-arragement for
that, but all but one sync_filesystem caller had the correct locking
already so that we could add that check there.  cachefiles grew
s_umount locking.

I've also added a WARN_ON to sync_filesystem to assert this for
future callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 0 insertions, 3 deletions

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 52d84522c2c2..9f179d4832d5 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -394,9 +394,6 @@ int btrfs_sync_fs(struct super_block *sb, int wait)
         struct btrfs_root *root = btrfs_sb(sb);
         int ret;
 
-        if (sb->s_flags & MS_RDONLY)
-                return 0;
-
         if (!wait) {
                 filemap_flush(root->fs_info->btree_inode->i_mapping);
                 return 0;