Re: [patch-final] Re: patch] cpusets, cgroups: disallow attachingkthreadd

[David Rientjes]

On Wed, 19 Oct 2011, Mike Galbraith 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.
> 
> Signed-off-by: Mike Galbraith <efault@xxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Acked-by: Paul Menage <paul@xxxxxxxxxxxxxx>
> 

Let's add Andrew to the cc so we can get it in -mm, I haven't seen it hit 
linux-next yet.

> ---
>  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;
--
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