sched,freezer: Remove unnecessary warning in __thaw_task

commit 9beb8c5 upstream.

Commit cff5f49 ("cgroup_freezer: cgroup_freezing: Check if not
frozen") modified the cgroup_freezing() logic to verify that the FROZEN
flag is not set, affecting the return value of the freezing() function,
in order to address a warning in __thaw_task.

A race condition exists that may allow tasks to escape being frozen. The
following scenario demonstrates this issue:

CPU 0 (get_signal path)		CPU 1 (freezer.state reader)
try_to_freeze			read freezer.state
__refrigerator			freezer_read
				update_if_frozen
WRITE_ONCE(current->__state, TASK_FROZEN);
				...
				/* Task is now marked frozen */
				/* frozen(task) == true */
				/* Assuming other tasks are frozen */
				freezer->state |= CGROUP_FROZEN;
/* freezing(current) returns false */
/* because cgroup is frozen (not freezing) */
break out
__set_current_state(TASK_RUNNING);
/* Bug: Task resumes running when it should remain frozen */

The existing !frozen(p) check in __thaw_task makes the
WARN_ON_ONCE(freezing(p)) warning redundant. Removing this warning enables
reverting commit cff5f49 ("cgroup_freezer: cgroup_freezing: Check if
not frozen") to resolve the issue.

This patch removes the warning from __thaw_task. A subsequent patch will
revert commit cff5f49 ("cgroup_freezer: cgroup_freezing: Check if
not frozen") to complete the fix.

Reported-by: Zhong Jiawei<[email protected]>
Signed-off-by: Chen Ridong <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Chen Ridong

kernel/freezer.c

@@ -201,18 +201,9 @@ static int __restore_freezer_state(struct task_struct *p, void *arg)
 
 void __thaw_task(struct task_struct *p)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(&freezer_lock, flags);
-	if (WARN_ON_ONCE(freezing(p)))
-		goto unlock;
-
-	if (!frozen(p) || task_call_func(p, __restore_freezer_state, NULL))
-		goto unlock;
-
-	wake_up_state(p, TASK_FROZEN);
-unlock:
-	spin_unlock_irqrestore(&freezer_lock, flags);
+	guard(spinlock_irqsave)(&freezer_lock);
+	if (frozen(p) && !task_call_func(p, __restore_freezer_state, NULL))
+		wake_up_state(p, TASK_FROZEN);
 }
 
 /**