cgroup: deprecate remount option changes

This patch marks the following features for deprecation.

* Rebinding subsys by remount: Never reached useful state - only works
  on empty hierarchies.

* release_agent update by remount: release_agent itself will be
  replaced with conventional fsnotify notification.

v2: Lennart pointed out that "name=" is necessary for mounts w/o any
    controller attached.  Drop "name=" deprecation.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizf@cn.fujitsu.com>
Cc: Lennart Poettering <mzxreary@0pointer.de>

Documentation/feature-removal-schedule.txt

@@ -539,3 +539,12 @@ Why:	There appear to be no production users of the get_robust_list syscall,
 	of ASLR. It was only ever intended for debugging, so it should be
 	removed.
 Who:	Kees Cook <keescook@chromium.org>
+
+----------------------------
+
+What:	cgroup option updates via remount
+When:	March 2013
+Why:	Remount currently allows changing bound subsystems and
+	release_agent.  Rebinding is hardly useful as it only works
+	when the hierarchy is empty and release_agent itself should be
+	replaced with conventional fsnotify.

kernel/cgroup.c

@@ -1294,6 +1294,11 @@ static int cgroup_remount(struct super_block *sb, int *flags, char *data)
 	if (ret)
 		goto out_unlock;
 
+	/* See feature-removal-schedule.txt */
+	if (opts.subsys_bits != root->actual_subsys_bits || opts.release_agent)
+		pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
+			   task_tgid_nr(current), current->comm);
+
 	/* Don't allow flags or name to change at remount */
 	if (opts.flags != root->flags ||
 	    (opts.name && strcmp(opts.name, root->name))) {