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, 2 insertions, 0 deletions

diff --git a/fs/cachefiles/interface.c b/fs/cachefiles/interface.c
index dafd484d7bda..431accd475a7 100644
--- a/fs/cachefiles/interface.c
+++ b/fs/cachefiles/interface.c
@@ -354,7 +354,9 @@ static void cachefiles_sync_cache(struct fscache_cache *_cache)
         /* make sure all pages pinned by operations on behalf of the netfs are
          * written to disc */
         cachefiles_begin_secure(cache, &saved_cred);
+        down_read(&cache->mnt->mnt_sb->s_umount);
         ret = sync_filesystem(cache->mnt->mnt_sb);
+        up_read(&cache->mnt->mnt_sb->s_umount);
         cachefiles_end_secure(cache, saved_cred);
 
         if (ret == -EIO)