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SubjectRe: [resubmit] Re: [patch-final] Re: patch] cpusets, cgroups: disallow attaching kthreadd
On Wed, 7 Dec 2011, Mike Galbraith wrote:

> Door #1 with speeling correction.
>
> 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>

Yes, this looks good. Andrew could you apply this?

> ---
> kernel/cpuset.c | 6 ++++--
> kernel/sched/core.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.
> @@ -1385,9 +1386,10 @@ 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 its
> + * 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/core.c
> ===================================================================
> --- linux-3.0-tip.orig/kernel/sched/core.c
> +++ linux-3.0-tip/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>
> @@ -7461,6 +7462,15 @@ 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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