cgroup: fix too early usage of static_branch_disable()

49d1dc4b81 ("cgroup: implement static_key based
cgroup_subsys_enabled() and cgroup_subsys_on_dfl()") converted cgroup
enabled test to use static_key; however, cgroup_disable() is called
before static_key subsystem itself is initialized and thus leads to
the following warning when "cgroup_disable=" parameter is specified.

 WARNING: CPU: 0 PID: 0 at kernel/jump_label.c:99 static_key_slow_dec+0x44/0x60()
 static_key_slow_dec used before call to jump_label_init
 ...
 Call Trace:
  [<ffffffff813b18c2>] dump_stack+0x44/0x62
  [<ffffffff8108dd52>] warn_slowpath_common+0x82/0xc0
  [<ffffffff8108ddec>] warn_slowpath_fmt+0x5c/0x80
  [<ffffffff8119c054>] static_key_slow_dec+0x44/0x60
  [<ffffffff81d826b6>] cgroup_disable+0xaf/0xd6
  [<ffffffff81d5f9de>] unknown_bootoption+0x8c/0x194
  [<ffffffff810b0c03>] parse_args+0x273/0x4a0
  [<ffffffff81d5fd67>] start_kernel+0x205/0x4b8
 ...

Fix it by making cgroup_disable() to record the subsystems to disable
in cgroup_disable_mask and moving the actual application to
cgroup_init() which is late enough and where the enabled state is
first used.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Andrey Wagin <avagin@gmail.com>
Link: http://lkml.kernel.org/g/CANaxB-yFuS4SA2znSvcKrO9L_CbHciHYW+o9bN8sZJ8eR9FxYA@mail.gmail.com
Fixes: 49d1dc4b81

kernel/cgroup.c

@@ -5124,6 +5124,8 @@ int __init cgroup_init_early(void)
 	return 0;
 }
 
+static unsigned long cgroup_disable_mask __initdata;
+
 /**
  * cgroup_init - cgroup initialization
  *
@@ -5170,8 +5172,12 @@ int __init cgroup_init(void)
 		 * disabled flag and cftype registration needs kmalloc,
 		 * both of which aren't available during early_init.
 		 */
-		if (!cgroup_ssid_enabled(ssid))
+		if (cgroup_disable_mask & (1 << ssid)) {
+			static_branch_disable(cgroup_subsys_enabled_key[ssid]);
+			printk(KERN_INFO "Disabling %s control group subsystem\n",
+			       ss->name);
 			continue;
+		}
 
 		cgrp_dfl_root.subsys_mask |= 1 << ss->id;
 
@@ -5595,11 +5601,7 @@ static int __init cgroup_disable(char *str)
 			if (strcmp(token, ss->name) &&
 			    strcmp(token, ss->legacy_name))
 				continue;
-
-			static_branch_disable(cgroup_subsys_enabled_key[i]);
-			printk(KERN_INFO "Disabling %s control group subsystem\n",
-			       ss->name);
-			break;
+			cgroup_disable_mask |= 1 << i;
 		}
 	}
 	return 1;