cgroup: inline cgroup_task_freeze()

After the introduction of the cgroup.kill there is only one call site
of cgroup_task_freeze() left: cgroup_exit(). cgroup_task_freeze() is
currently taking rcu_read_lock() to read task's cgroup flags, but
because it's always called with css_set_lock locked, the rcu protection
is excessive.

Simplify the code by inlining cgroup_task_freeze().

v2: fix build

Signed-off-by: Roman Gushchin <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: rgushchin

include/linux/cgroup.h

@@ -906,20 +906,6 @@ void cgroup_freeze(struct cgroup *cgrp, bool freeze);
 void cgroup_freezer_migrate_task(struct task_struct *task, struct cgroup *src,
 				 struct cgroup *dst);
 
-static inline bool cgroup_task_freeze(struct task_struct *task)
-{
-	bool ret;
-
-	if (task->flags & PF_KTHREAD)
-		return false;
-
-	rcu_read_lock();
-	ret = test_bit(CGRP_FREEZE, &task_dfl_cgroup(task)->flags);
-	rcu_read_unlock();
-
-	return ret;
-}
-
 static inline bool cgroup_task_frozen(struct task_struct *task)
 {
 	return task->frozen;
@@ -929,10 +915,6 @@ static inline bool cgroup_task_frozen(struct task_struct *task)
 
 static inline void cgroup_enter_frozen(void) { }
 static inline void cgroup_leave_frozen(bool always_leave) { }
-static inline bool cgroup_task_freeze(struct task_struct *task)
-{
-	return false;
-}
 static inline bool cgroup_task_frozen(struct task_struct *task)
 {
 	return false;

kernel/cgroup/cgroup.c

@@ -6267,7 +6267,8 @@ void cgroup_exit(struct task_struct *tsk)
 	cset->nr_tasks--;
 
 	WARN_ON_ONCE(cgroup_task_frozen(tsk));
-	if (unlikely(cgroup_task_freeze(tsk)))
+	if (unlikely(!(tsk->flags & PF_KTHREAD) &&
+		     test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))
 		cgroup_update_frozen(task_dfl_cgroup(tsk));
 
 	spin_unlock_irq(&css_set_lock);