[PATCH] cgroup,freezer: hold cpu_hotplug_lock before freezer_mutex in freezer_css_{online,offline}()

[Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>]

syzbot is again reporting circular locking dependency between
cpu_hotplug_lock and freezer_mutex. Do like what we did with
commit 57dcd64c7e036299 ("cgroup,freezer: hold cpu_hotplug_lock
before freezer_mutex").

Reported-by: syzbot <syzbot+2ab700fe1829880a2ec6@xxxxxxxxxxxxxxxxxxxxxxxxx>
Closes: https://syzkaller.appspot.com/bug?extid=2ab700fe1829880a2ec6
Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Tested-by: syzbot <syzbot+2ab700fe1829880a2ec6@xxxxxxxxxxxxxxxxxxxxxxxxx>
---
 kernel/cgroup/legacy_freezer.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/cgroup/legacy_freezer.c b/kernel/cgroup/legacy_freezer.c
index 936473203a6b..122dacb3a443 100644
--- a/kernel/cgroup/legacy_freezer.c
+++ b/kernel/cgroup/legacy_freezer.c
@@ -108,16 +108,18 @@ static int freezer_css_online(struct cgroup_subsys_state *css)
 	struct freezer *freezer = css_freezer(css);
 	struct freezer *parent = parent_freezer(freezer);
 
+	cpus_read_lock();
 	mutex_lock(&freezer_mutex);
 
 	freezer->state |= CGROUP_FREEZER_ONLINE;
 
 	if (parent && (parent->state & CGROUP_FREEZING)) {
 		freezer->state |= CGROUP_FREEZING_PARENT | CGROUP_FROZEN;
-		static_branch_inc(&freezer_active);
+		static_branch_inc_cpuslocked(&freezer_active);
 	}
 
 	mutex_unlock(&freezer_mutex);
+	cpus_read_unlock();
 	return 0;
 }
 
@@ -132,14 +134,16 @@ static void freezer_css_offline(struct cgroup_subsys_state *css)
 {
 	struct freezer *freezer = css_freezer(css);
 
+	cpus_read_lock();
 	mutex_lock(&freezer_mutex);
 
 	if (freezer->state & CGROUP_FREEZING)
-		static_branch_dec(&freezer_active);
+		static_branch_dec_cpuslocked(&freezer_active);
 
 	freezer->state = 0;
 
 	mutex_unlock(&freezer_mutex);
+	cpus_read_unlock();
 }
 
 static void freezer_css_free(struct cgroup_subsys_state *css)
-- 
2.18.4