cgroup: Explicitly remove core interface files

The "cgroup." core interface files bypass the usual interface removal
path and get removed recursively along with the cgroup itself.  While
this works now, the subtle discrepancy gets in the way of implementing
common mechanisms.

This patch updates cgroup core interface file handling so that it's
consistent with controller interface files.  When added, the css is
marked CSS_VISIBLE and they're explicitly removed before the cgroup is
destroyed.

This doesn't cause user-visible behavior changes.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -1573,8 +1573,17 @@ static void css_clear_dir(struct cgroup_subsys_state *css)
 
 	css->flags &= ~CSS_VISIBLE;
 
+	if (!css->ss) {
+		if (cgroup_on_dfl(cgrp))
+			cfts = cgroup_base_files;
+		else
+			cfts = cgroup1_base_files;
+
+		cgroup_addrm_files(css, cgrp, cfts, false);
+	} else {
 		list_for_each_entry(cfts, &css->ss->cfts, node)
 			cgroup_addrm_files(css, cgrp, cfts, false);
+	}
 }
 
 /**
@@ -1598,9 +1607,10 @@ static int css_populate_dir(struct cgroup_subsys_state *css)
 		else
 			cfts = cgroup1_base_files;
 
-		return cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
-	}
-
+		ret = cgroup_addrm_files(&cgrp->self, cgrp, cfts, true);
+		if (ret < 0)
+			return ret;
+	} else {
 		list_for_each_entry(cfts, &css->ss->cfts, node) {
 			ret = cgroup_addrm_files(css, cgrp, cfts, true);
 			if (ret < 0) {
@@ -1608,6 +1618,7 @@ static int css_populate_dir(struct cgroup_subsys_state *css)
 				goto err;
 			}
 		}
+	}
 
 	css->flags |= CSS_VISIBLE;
 
@@ -5090,10 +5101,8 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	for_each_css(css, ssid, cgrp)
 		kill_css(css);
 
-	/*
-	 * Remove @cgrp directory along with the base files.  @cgrp has an
-	 * extra ref on its kn.
-	 */
+	/* clear and remove @cgrp dir, @cgrp has an extra ref on its kn */
+	css_clear_dir(&cgrp->self);
 	kernfs_remove(cgrp->kn);
 
 	if (parent && cgroup_is_threaded(cgrp))