Re: [resubmit] Re: [patch-final] Re: patch] cpusets, cgroups: disallow attaching kthreadd

[Mike Galbraith <efault@gmx.de>]

On Fri, 2012-01-06 at 14:06 -0800, Andrew Morton wrote:
> On Wed, 07 Dec 2011 05:36:57 +0100
> Mike Galbraith <efault@gmx.de> wrote:
> 
> > cpusets, cgroups: disallow attaching kthreadd
> > 
> > Allowing kthreadd to be moved to a non-root group makes no sense, it being
> > a global resource, and needlessly leads unsuspecting users toward trouble.
> > 
> > 1. An RT workqueue worker thread spawned in a task group with no rt_runtime
> > allocated is not schedulable.  Simple user error, but harmful to the box.
> > 
> > 2. A worker thread which acquires PF_THREAD_BOUND can never leave a cpuset,
> > rendering the cpuset immortal.
> > 
> > Save the user some unexpected trouble, just say no.
> 
> Someone's been screwing around in linux-next during the merge window. 
> afacit some patch which was previously there has magically disappeared.
> That killed your patch - the original version of the
> kernel/sched/core.c chagne applies to the new linux-next but the
> kernel/cpuset.c part is all wrecked.

Hm, what to do...

I can't get to mmotm to see what's there, but fresh pulled linux-next
has it as 904655b9, and looked ok. (?)  The only thing I can think to do
is post it as just plugged into fresh pulled master and hope that helps
turn little floundering guppy right side up again.

From: Mike Galbraith <efault@gmx.de>

cpusets, cgroups: disallow attaching kthreadd

Allowing kthreadd to be moved to a non-root group makes no sense, it being
a global resource, and needlessly leads unsuspecting users toward trouble.

1. An RT workqueue worker thread spawned in a task group with no
   rt_runtime allocated is not schedulable.  Simple user error, but
   harmful to the box.

2. A worker thread which acquires PF_THREAD_BOUND can never leave a
   cpuset, rendering the cpuset immortal.

Save the user some unexpected trouble, just say no.

Signed-off-by: Mike Galbraith <efault@gmx.de>
Acked-by: David Rientjes <rientjes@google.com>
Acked-by: Paul Menage <paul@paulmenage.org>
Cc: Tejun Heo <htejun@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 0b1712d..cf438f3 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -59,6 +59,7 @@
 #include <linux/mutex.h>
 #include <linux/workqueue.h>
 #include <linux/cgroup.h>
+#include <linux/kthread.h>
 
 /*
  * Workqueue for cpuset related tasks.
@@ -1404,9 +1405,10 @@ static int cpuset_can_attach(struct cgroup_subsys *ss, struct cgroup *cont,
 	 * set of allowed nodes is unnecessary.  Thus, cpusets are not
 	 * applicable for such threads.  This prevents checking for success of
 	 * set_cpus_allowed_ptr() on all attached tasks before cpus_allowed may
-	 * be changed.
+	 * be changed.  We also disallow attaching kthreadd, to prevent a child
+	 * from becoming trapped should it later acquire PF_THREAD_BOUND.
 	 */
-	if (tsk->flags & PF_THREAD_BOUND)
+	if (tsk->flags & PF_THREAD_BOUND || tsk == kthreadd_task)
 		return -EINVAL;
 
 	return 0;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 4dbfd04..165d610 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -71,6 +71,7 @@
 #include <linux/ftrace.h>
 #include <linux/slab.h>
 #include <linux/init_task.h>
+#include <linux/kthread.h>
 
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
@@ -7576,6 +7577,15 @@ cpu_cgroup_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 	if (tsk->sched_class != &fair_sched_class)
 		return -EINVAL;
 #endif
+	/*
+	 * kthreadd can fork workers for an RT workqueue in a cgroup
+	 * which may or may not have rt_runtime allocated.  Just say no,
+	 * as attaching a global resource to a non-root group  doesn't
+	 * make any sense anyway.
+	 */
+	if (tsk == kthreadd_task)
+		return -EINVAL;
+
 	return 0;
 }