cgroup: initialize cgrp_dfl_root_inhibit_ss_mask from !->dfl_files test

cgrp_dfl_root_inhibit_ss_mask determines which subsystems are not
supported on the default hierarchy and is currently initialized
statically and just includes the debug subsystem.  Now that there's
cgroup_subsys->dfl_files, we can easily tell which subsystems support
the default hierarchy or not.

Let's initialize cgrp_dfl_root_inhibit_ss_mask by testing whether
cgroup_subsys->dfl_files is NULL.  After all, subsystems with NULL
->dfl_files aren't useable on the default hierarchy anyway.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -156,11 +156,7 @@ static bool cgrp_dfl_root_visible;
 static bool cgroup_legacy_files_on_dfl;
 
 /* some controllers are not supported in the default hierarchy */
-static const unsigned int cgrp_dfl_root_inhibit_ss_mask = 0
-#ifdef CONFIG_CGROUP_DEBUG
-	| (1 << debug_cgrp_id)
-#endif
-	;
+static unsigned int cgrp_dfl_root_inhibit_ss_mask;
 
 /* The list of hierarchy roots */
 
@@ -4939,6 +4935,9 @@ int __init cgroup_init(void)
 		if (cgroup_legacy_files_on_dfl && !ss->dfl_cftypes)
 			ss->dfl_cftypes = ss->legacy_cftypes;
 
+		if (!ss->dfl_cftypes)
+			cgrp_dfl_root_inhibit_ss_mask |= 1 << ss->id;
+
 		if (ss->dfl_cftypes == ss->legacy_cftypes) {
 			WARN_ON(cgroup_add_cftypes(ss, ss->dfl_cftypes));
 		} else {