freezer: Clarify that only cgroup1 freezer uses PM freezer

cgroup1 freezer piggybacks on the PM freezer, which inadvertently allowed
userspace to produce uninterruptible tasks at will. To avoid the issue,
cgroup2 freezer switched to a separate job control based mechanism. While
this happened a long time ago, the code and comment haven't been updated
making it confusing to people who aren't familiar with the history.

Rename cgroup_freezing() to cgroup1_freezing() and update comments on top of
freezing() and frozen() to clarify that cgroup2 freezer isn't covered by the
PM freezer mechanism.

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Qu Wenruo <wqu@suse.com>
Link: https://patch.msgid.link/aPZ3q6Hm865NicBC@slm.duckdns.org
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: htejun

include/linux/freezer.h

@@ -22,14 +22,18 @@ extern bool pm_nosig_freezing;		/* PM nosig freezing in effect */
 extern unsigned int freeze_timeout_msecs;
 
 /*
- * Check if a process has been frozen
+ * Check if a process has been frozen for PM or cgroup1 freezer. Note that
+ * cgroup2 freezer uses the job control mechanism and does not interact with
+ * the PM freezer.
  */
 extern bool frozen(struct task_struct *p);
 
 extern bool freezing_slow_path(struct task_struct *p);
 
 /*
- * Check if there is a request to freeze a process
+ * Check if there is a request to freeze a task from PM or cgroup1 freezer.
+ * Note that cgroup2 freezer uses the job control mechanism and does not
+ * interact with the PM freezer.
  */
 static inline bool freezing(struct task_struct *p)
 {
@@ -63,9 +67,9 @@ extern bool freeze_task(struct task_struct *p);
 extern bool set_freezable(void);
 
 #ifdef CONFIG_CGROUP_FREEZER
-extern bool cgroup_freezing(struct task_struct *task);
+extern bool cgroup1_freezing(struct task_struct *task);
 #else /* !CONFIG_CGROUP_FREEZER */
-static inline bool cgroup_freezing(struct task_struct *task)
+static inline bool cgroup1_freezing(struct task_struct *task)
 {
 	return false;
 }

kernel/cgroup/legacy_freezer.c

@@ -63,7 +63,7 @@ static struct freezer *parent_freezer(struct freezer *freezer)
 	return css_freezer(freezer->css.parent);
 }
 
-bool cgroup_freezing(struct task_struct *task)
+bool cgroup1_freezing(struct task_struct *task)
 {
 	bool ret;
 

kernel/freezer.c

@@ -44,7 +44,7 @@ bool freezing_slow_path(struct task_struct *p)
 	if (tsk_is_oom_victim(p))
 		return false;
 
-	if (pm_nosig_freezing || cgroup_freezing(p))
+	if (pm_nosig_freezing || cgroup1_freezing(p))
 		return true;
 
 	if (pm_freezing && !(p->flags & PF_KTHREAD))