Re: patch] cpusets, cgroups: disallow attaching kthreadd

[Mike Galbraith]

On Tue, 2011-10-18 at 11:23 +0200, Mike Galbraith wrote:
> On Tue, 2011-10-18 at 11:06 +0200, Peter Zijlstra wrote:
> > On Tue, 2011-10-18 at 10:47 +0200, Mike Galbraith wrote:
> > > On Tue, 2011-10-18 at 10:42 +0200, Peter Zijlstra wrote:
> > > 
> > > > Why special case these two, why not simply disallow moving kthreadd into
> > > > _any_ cgroup what so ever and be done with it?
> > > 
> > > These were the cases where it hurt, and cpusets don't have to be a
> > > component of a cgroup.  So we'd still need the cpuset bit.
> > 
> > but but but,, cpusetes are a cgroup thingy?!
> 
> By golly, mount -t cpuset and the thing does say type cgroup.  How 'bout
> that, learn something new every day.  Dinkier is better.

From: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>

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>

---
 kernel/cgroup.c |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: linux-3.0-tip/kernel/cgroup.c
===================================================================
--- linux-3.0-tip.orig/kernel/cgroup.c
+++ linux-3.0-tip/kernel/cgroup.c
@@ -60,6 +60,7 @@
 #include <linux/eventfd.h>
 #include <linux/poll.h>
 #include <linux/flex_array.h> /* used in cgroup_attach_proc */
+#include <linux/kthread.h>
 
 #include <linux/atomic.h>
 
@@ -1824,6 +1825,14 @@ int cgroup_attach_task(struct cgroup *cg
 	struct cgroup *oldcgrp;
 	struct cgroupfs_root *root = cgrp->root;
 
+	/*
+	 * Workqueue threads may acquire PF_THREAD_BOUND and become
+	 * trapped in a cpuset, or RT worker may be born in a cgroup
+	 * with no rt_runtime allocated.  Just say no.
+	 */
+	if (tsk == kthreadd_task)
+		return -EINVAL;
+
 	/* Nothing to do if the task is already in that cgroup */
 	oldcgrp = task_cgroup_from_root(tsk, root);
 	if (cgrp == oldcgrp)


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