patch] cpusets, cgroups: disallow attaching kthreadd

[Mike Galbraith <efault@gmx.de>]

On Mon, 2011-10-10 at 10:03 +0200, Mike Galbraith wrote:
> On Mon, 2011-10-10 at 07:34 +0200, Mike Galbraith wrote:
> > Maybe the below (which seems to have been stillborn) should be done to
> > cgroups as well.  Postmortem: kthreadd is attached to a cgroup with no
> > rt_runtime allocated, gives birth to severely handicapped kstop threads,
> > humongous crash dump follows.
> > 
> > Fiddling with kthreadd is user error, but since it makes no sense to
> > move the thing, why not just say no, and save the user's toes some
> > needless wear and tear.
> > 
> > > If cpusets doesn't want to let PF_THREAD_BOUND threads out, how about
> > > cpusets not letting userland scripts attach kthreadd instead?
> > > 
> > > cpusets: disallow moving kthreadd into a cpuset.
> 
> So how about this, both dirt simple and effective.

Ahem, the buildable copy..

I've been asked to add this toe saver to the trees I work on at least
until something better comes along.  ACK/NACK equally understandable
given the infrequency of the toe shooting, and user's right to do so.

Off you go, fly or die little patchlet.

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>

---
 kernel/cpuset.c |    6 ++++--
 kernel/sched.c  |   10 ++++++++++
 2 files changed, 14 insertions(+), 2 deletions(-)

Index: linux-3.0-tip/kernel/cpuset.c
===================================================================
--- linux-3.0-tip.orig/kernel/cpuset.c
+++ linux-3.0-tip/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.
@@ -1382,9 +1383,10 @@ static int cpuset_can_attach(struct cgro
 	 * 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 it's
+	 * child from becoming trapped should it then acquire PF_THREAD_BOUND.
 	 */
-	if (tsk->flags & PF_THREAD_BOUND)
+	if (tsk->flags & PF_THREAD_BOUND || tsk == kthreadd_task)
 		return -EINVAL;
 
 	return 0;
Index: linux-3.0-tip/kernel/sched.c
===================================================================
--- linux-3.0-tip.orig/kernel/sched.c
+++ linux-3.0-tip/kernel/sched.c
@@ -71,6 +71,7 @@
 #include <linux/ctype.h>
 #include <linux/ftrace.h>
 #include <linux/slab.h>
+#include <linux/kthread.h>
 
 #include <asm/tlb.h>
 #include <asm/irq_regs.h>
@@ -9141,6 +9142,15 @@ cpu_cgroup_destroy(struct cgroup_subsys
 static int
 cpu_cgroup_can_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
+	/*
+	 * 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;
+
 #ifdef CONFIG_RT_GROUP_SCHED
 	if (!sched_rt_can_attach(cgroup_tg(cgrp), tsk))
 		return -EINVAL;