cgroup: get rid of cgroup_freezer_frozen_exit()

A task should never enter the exit path with the task->frozen bit set.
Any frozen task must enter the signal handling loop and the only
way to escape is through cgroup_leave_frozen(true), which
unconditionally drops the task->frozen bit. So it means that
cgroyp_freezer_frozen_exit() has zero chances to be called and
has to be removed.

Let's put a WARN_ON_ONCE() instead of the cgroup_freezer_frozen_exit()
call to catch any potential leak of the task's frozen bit.
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Roman Gushchin <guro@fb.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -889,7 +889,7 @@ void cgroup_update_frozen(struct cgroup *cgrp);
 void cgroup_freeze(struct cgroup *cgrp, bool freeze);
 void cgroup_freezer_migrate_task(struct task_struct *task, struct cgroup *src,
 				 struct cgroup *dst);
-void cgroup_freezer_frozen_exit(struct task_struct *task);
+
 static inline bool cgroup_task_freeze(struct task_struct *task)
 {
 	bool ret;

kernel/cgroup/cgroup.c

@@ -5926,9 +5926,8 @@ void cgroup_exit(struct task_struct *tsk)
 		css_set_move_task(tsk, cset, NULL, false);
 		cset->nr_tasks--;
 
-		if (unlikely(cgroup_task_frozen(tsk)))
-			cgroup_freezer_frozen_exit(tsk);
-		else if (unlikely(cgroup_task_freeze(tsk)))
+		WARN_ON_ONCE(cgroup_task_frozen(tsk));
+		if (unlikely(cgroup_task_freeze(tsk)))
 			cgroup_update_frozen(task_dfl_cgroup(tsk));
 
 		spin_unlock_irq(&css_set_lock);

kernel/cgroup/freezer.c

@@ -248,16 +248,6 @@ void cgroup_freezer_migrate_task(struct task_struct *task,
 	cgroup_freeze_task(task, test_bit(CGRP_FREEZE, &dst->flags));
 }
 
-void cgroup_freezer_frozen_exit(struct task_struct *task)
-{
-	struct cgroup *cgrp = task_dfl_cgroup(task);
-
-	lockdep_assert_held(&css_set_lock);
-
-	cgroup_dec_frozen_cnt(cgrp);
-	cgroup_update_frozen(cgrp);
-}
-
 void cgroup_freeze(struct cgroup *cgrp, bool freeze)
 {
 	struct cgroup_subsys_state *css;