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SubjectRe: [patch] cpusets, cgroups: disallow attaching kthreadd
Hello,

(cc'ing Thomas and Steven and quoting whole body)

On Mon, Oct 10, 2011 at 10:03:34AM +0200, Mike Galbraith wrote:
> On Mon, 2011-10-10 at 07:34 +0200, Mike Galbraith wrote:
> > Maybe the below (which seems to have been stillborn) should be done to
> > cgroups as well. Postmortem: kthreadd is attached to a cgroup with no
> > rt_runtime allocated, gives birth to severely handicapped kstop threads,
> > humongous crash dump follows.
> >
> > Fiddling with kthreadd is user error, but since it makes no sense to
> > move the thing, why not just say no, and save the user's toes some
> > needless wear and tear.
> >
> > > If cpusets doesn't want to let PF_THREAD_BOUND threads out, how about
> > > cpusets not letting userland scripts attach kthreadd instead?
> > >
> > > cpusets: disallow moving kthreadd into a cpuset.
>
> So how about this, both dirt simple and effective.
>
> 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>

Yes, I think we need something like this. wq workers were using
PF_THREAD_BOUND to prevent diddling from userland which made some
unhappy. Maybe we need a flag to properly indicate "don't diddle with
this thread from userland"? But, then, mainline kernel wouldn't need
the current PF_THREAD_BOUND at all. Peter, Steven, what do you think?

Thanks.

> ---
> kernel/cpuset.c | 6 ++++--
> kernel/sched.c | 9 +++++++++
> 2 files changed, 13 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
> @@ -9132,6 +9132,15 @@ 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;
>
>

--
tejun


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