sched: Fix more TASK_state comparisons

Boris reported hung_task splats after commit 5aec788 ("sched: Fix
TASK_state comparisons"). Upon closer consideration of that change it
doesn't only exclude TASK_KILLABLE, but also TASK_IDLE.

Update the comment to reflect this fact and add an additional
TASK_NOLOAD test to exclude them.

Additionally, remove the TASK_FREEZABLE early exit from
check_hung_task(), a freezable task is not a frozen task.

Fixes: 5aec788 ("sched: Fix TASK_state comparisons")
Reported-by: Borislav Petkov <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Tested-by: Borislav Petkov <[email protected]>

Author: Peter Zijlstra

kernel/hung_task.c

@@ -95,7 +95,7 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
 	 * Ensure the task is not frozen.
 	 * Also, skip vfork and any other user process that freezer should skip.
 	 */
-	if (unlikely(READ_ONCE(t->__state) & (TASK_FREEZABLE | TASK_FROZEN)))
+	if (unlikely(READ_ONCE(t->__state) & TASK_FROZEN))
 		return;
 
 	/*
@@ -200,10 +200,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
 				goto unlock;
 			last_break = jiffies;
 		}
-		/* skip the TASK_KILLABLE tasks -- these can be killed */
+		/*
+		 * skip the TASK_KILLABLE tasks -- these can be killed
+		 * skip the TASK_IDLE tasks -- those are genuinely idle
+		 */
 		state = READ_ONCE(t->__state);
 		if ((state & TASK_UNINTERRUPTIBLE) &&
-		    !(state & TASK_WAKEKILL))
+		    !(state & TASK_WAKEKILL) &&
+		    !(state & TASK_NOLOAD))
 			check_hung_task(t, timeout);
 	}
  unlock: