freezer, sched: Report frozen tasks as 'D' instead of 'R'

[ Upstream commit f718faf3940e95d5d34af9041f279f598396ab7d ]

Before commit:

  f5d39b0 ("freezer,sched: Rewrite core freezer logic")

the frozen task stat was reported as 'D' in cgroup v1.

However, after rewriting the core freezer logic, the frozen task stat is
reported as 'R'. This is confusing, especially when a task with stat of
'S' is frozen.

This bug can be reproduced with these steps:

	$ cd /sys/fs/cgroup/freezer/
	$ mkdir test
	$ sleep 1000 &
	[1] 739         // task whose stat is 'S'
	$ echo 739 > test/cgroup.procs
	$ echo FROZEN > test/freezer.state
	$ ps -aux | grep 739
	root     739  0.1  0.0   8376  1812 pts/0    R    10:56   0:00 sleep 1000

As shown above, a task whose stat is 'S' was changed to 'R' when it was
frozen.

To solve this regression, simply maintain the same reported state as
before the rewrite.

[ mingo: Enhanced the changelog and comments ]

Fixes: f5d39b0 ("freezer,sched: Rewrite core freezer logic")
Signed-off-by: Chen Ridong <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Tejun Heo <[email protected]>
Acked-by: Michal Koutný <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sasha Levin <[email protected]>

Author: Chen Ridong

include/linux/sched.h

@@ -1670,8 +1670,9 @@ static inline unsigned int __task_state_index(unsigned int tsk_state,
 	 * We're lying here, but rather than expose a completely new task state
 	 * to userspace, we can make this appear as if the task has gone through
 	 * a regular rt_mutex_lock() call.
+	 * Report frozen tasks as uninterruptible.
 	 */
-	if (tsk_state & TASK_RTLOCK_WAIT)
+	if ((tsk_state & TASK_RTLOCK_WAIT) || (tsk_state & TASK_FROZEN))
 		state = TASK_UNINTERRUPTIBLE;
 
 	return fls(state);