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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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