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    SubjectRe: [patch] cgroups: disallow attaching kthreadd
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    On Fri, 2012-04-20 at 10:57 -0700, Tejun Heo wrote: 
    > Hello, Mike.
    >
    > This is dragging out too long but can you please update the patch so
    > that both kthreadd and PF_THREAD_BOUND tasks are disallowed from !root
    > cgroups?

    cgroups: disallow attaching kthreadd or PF_THREAD_BOUND threads

    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 <mgalbraith@suse.de>
    Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Acked-by: Thomas Gleixner <tglx@linutronix.de>
    ---
    kernel/cgroup.c | 12 ++++++++++++
    1 file changed, 12 insertions(+)

    --- a/kernel/cgroup.c
    +++ b/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>

    @@ -2172,6 +2173,17 @@ static int attach_task_by_pid(struct cgr

    if (threadgroup)
    tsk = tsk->group_leader;
    +
    + /*
    + * 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 || (tsk->flags & PF_THREAD_BOUND)) {
    + ret = -EINVAL;
    + goto out_unlock_cgroup;
    + }
    +
    get_task_struct(tsk);
    rcu_read_unlock();




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