[PATCH 1/4] fs: split update_time() into update_time() and write_time()

[Christoph Hellwig]

On Fri, Nov 21, 2014 at 02:59:21PM -0500, Theodore Ts'o wrote:
> We needed to preserve update_time() because btrfs wants to have a
> special btrfs_root_readonly() check; otherwise we could drop the
> update_time() inode operation entirely.

Can't btrfs just set the immutable flag on every inode that is read
when the root has the BTRFS_ROOT_SUBVOL_RDONLY flag?  That would
cut down the places that need this check to the ioctl path so that
we prevent users from clearling the immutable flag.