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

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Date:	Wed, 07 Dec 2011 05:36:57 +0100
From:	Mike Galbraith <efault@....de>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	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: [resubmit] Re: [patch-final] Re: patch] cpusets, cgroups:
 disallow attaching kthreadd

Door #1 with speeling correction.

From: Mike Galbraith <efault@....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@....de>
Acked-by: David Rientjes <rientjes@...gle.com>
Acked-by: Paul Menage <paul@...lmenage.org>

---
 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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