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Subject[patch] cpusets, cgroups: disallow attaching kthreadd
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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.

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 | 9 +++++++++
2 files changed, 13 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
@@ -9132,6 +9132,15 @@ 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;



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