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/ubifs/super.c b/fs/ubifs/super.c
index e9f7a754c4f7..84f3c7fd1552 100644
--- a/fs/ubifs/super.c
+++ b/fs/ubifs/super.c
@@ -447,9 +447,6 @@ static int ubifs_sync_fs(struct super_block *sb, int wait)
         if (!wait)
                 return 0;
 
-        if (sb->s_flags & MS_RDONLY)
-                return 0;
-
         /*
          * VFS calls '->sync_fs()' before synchronizing all dirty inodes and
          * pages, so synchronize them first, then commit the journal. Strictly