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    Subject[patch-final] Re: patch] cpusets, cgroups: disallow attaching kthreadd
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    On Tue, 2011-10-18 at 21:57 -0700, Paul Menage wrote:
    > On Tue, Oct 18, 2011 at 1:10 AM, Mike Galbraith <efault@gmx.de> wrote:
    > >
    > > Save the user some unexpected trouble, just say no.
    > >
    > > Signed-off-by: Mike Galbraith <efault@gmx.de>
    >
    > Acked-by: Paul Menage <paul@paulmenage.org>

    Thanks. Ok, seems it is decided that this is the best way to deal with
    annoying corner-case. For convenience, patch with acquired acks below.


    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>
    Acked-by: David Rientjes <rientjes@google.com>
    Acked-by: Paul Menage <paul@paulmenage.org>

    ---
    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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