[patch 2/2] cpu_cgroup: disallow attaching kthreadd

[David Rientjes <rientjes@google.com>]

From: Mike Galbraith <efault@gmx.de>

An RT workqueue worker thread spawned in a cpu cgroup with no rt_runtime
allocated is not schedulable.  Simple user error, but harmful to the box.
Save the user some unexpected trouble, just say no.

Acked-by: Paul Menage <paul@paulmenage.org>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: David Rientjes <rientjes@google.com>
---
 kernel/sched/core.c |    8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -72,6 +72,7 @@
 #include <linux/slab.h>
 #include <linux/init_task.h>
 #include <linux/binfmts.h>
+#include <linux/kthread.h>
 
 #include <asm/switch_to.h>
 #include <asm/tlb.h>
@@ -7636,6 +7637,13 @@ static int cpu_cgroup_can_attach(struct cgroup *cgrp,
 		if (task->sched_class != &fair_sched_class)
 			return -EINVAL;
 #endif
+		/*
+		 * Disallow kthreadd since it can fork workers for an RT
+		 * workqueue in a cgroup which may or may not have rt_runtime
+		 * allocated.
+		 */
+		if (task == kthreadd_task)
+			return -EINVAL;
 	}
 	return 0;
 }