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

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Date:	Tue, 6 Dec 2011 12:13:10 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>
cc:	Paul Menage <paul@...lmenage.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Tejun Heo <htejun@...il.com>, Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch-final] Re: patch] cpusets, cgroups: disallow attaching
 kthreadd

On Thu, 10 Nov 2011, David Rientjes wrote:

> On Wed, 26 Oct 2011, David Rientjes wrote:
> 
> > 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@....de>
> > > Acked-by: David Rientjes <rientjes@...gle.com>
> > > Acked-by: Paul Menage <paul@...lmenage.org>
> > > 
> > 
> > Let's add Andrew to the cc so we can get it in -mm, I haven't seen it hit 
> > linux-next yet.
> > 
> 
> Ping?  Still missing from -mm and linux-next.
> 

Ping #2?

> > > ---
> > >  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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