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

From: Andrew Morton
Date: Wed Dec 14 2011 - 15:16:12 EST


On Wed, 7 Dec 2011 14:03:56 -0800 (PST)
David Rientjes <rientjes@xxxxxxxxxx> wrote:

> On Wed, 7 Dec 2011, Mike Galbraith wrote:
>
> > Door #1 with speeling correction.
> >
> > From: Mike Galbraith <efault@xxxxxx>
> >
> > 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@xxxxxx>
> > Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> > Acked-by: Paul Menage <paul@xxxxxxxxxxxxxx>
>
> Yes, this looks good. Andrew could you apply this?

This patch needed considerable rework resulting from ongoing churn in
linux-next cgroups code. It needs checking and retesting, please.


From: Mike Galbraith <efault@xxxxxx>
Subject: 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@xxxxxx>
Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
Acked-by: Paul Menage <paul@xxxxxxxxxxxxxx>
Cc: Tejun Heo <htejun@xxxxxxxxx>
Cc: Li Zefan <lizf@xxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---

kernel/cpuset.c | 7 +++++--
kernel/sched/core.c | 9 +++++++++
2 files changed, 14 insertions(+), 2 deletions(-)

diff -puN kernel/cpuset.c~cpusets-cgroups-disallow-attaching-kthreadd kernel/cpuset.c
--- a/kernel/cpuset.c~cpusets-cgroups-disallow-attaching-kthreadd
+++ a/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.
@@ -1417,9 +1418,11 @@ static int cpuset_can_attach(struct cgro
* 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.
+ * cpus_allowed may be changed. We also disallow attaching
+ * kthreadd, to prevent its child from becoming trapped should
+ * it then acquire PF_THREAD_BOUND.
*/
- if (task->flags & PF_THREAD_BOUND)
+ if (tsk->flags & PF_THREAD_BOUND || tsk == kthreadd_task)
return -EINVAL;
if ((ret = security_task_setscheduler(task)))
return ret;
diff -puN kernel/sched/core.c~cpusets-cgroups-disallow-attaching-kthreadd kernel/sched/core.c
--- a/kernel/sched/core.c~cpusets-cgroups-disallow-attaching-kthreadd
+++ a/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>
@@ -7535,6 +7536,14 @@ static int cpu_cgroup_can_attach(struct
struct task_struct *task;

cgroup_taskset_for_each(task, cgrp, tset) {
+ /*
+ * 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), task))
return -EINVAL;
_

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