Re: 2.6.35-rc2-git1 - include/linux/cgroup.h:534 invoked rcu_dereference_check() without protection!

[Miles Lane]

On Tue, Jun 8, 2010 at 4:40 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, 2010-06-08 at 00:16 -0400, Miles Lane wrote:
>> On Mon, Jun 7, 2010 at 8:19 PM, Paul E. McKenney
>> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
>> > On Mon, Jun 07, 2010 at 02:14:25PM -0400, Miles Lane wrote:
>> >> Hi All,
>> >>
>> >> I just reproduced a warning I reported quite a while ago.  Is a patch
>> >> for this in the pipeline?
>> >
>> > I proposed a patch, thinking that it was a false positive.  Peter Zijlstra
>> > pointed out that there was a real race, and proposed an alternative patch,
>> > which may be found at http://lkml.org/lkml/2010/4/22/603.
>> >
>> > Could you please test Peter's patch and let us know if it cures the problem?
>> >
>
> Gah, this task_group() stuff is annoying, how about something like the
> below which teaches task_group() about the task_rq()->lock rule?
>
> ---
>  include/linux/cgroup.h |   20 +++++++++++----
>  kernel/sched.c         |   61 +++++++++++++++++++++++++----------------------
>  2 files changed, 46 insertions(+), 35 deletions(-)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 0c62160..1efd212 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -525,13 +525,21 @@ static inline struct cgroup_subsys_state *cgroup_subsys_state(
>        return cgrp->subsys[subsys_id];
>  }
>
> -static inline struct cgroup_subsys_state *task_subsys_state(
> -       struct task_struct *task, int subsys_id)
> +/*
> + * function to get the cgroup_subsys_state which allows for extra
> + * rcu_dereference_check() conditions, such as locks used during the
> + * cgroup_subsys::attach() methods.
> + */
> +#define task_subsys_state_check(task, subsys_id, __c)                  \
> +       rcu_dereference_check(task->cgroups->subsys[subsys_id],         \
> +                             rcu_read_lock_held() ||                   \
> +                             lockdep_is_held(&task->alloc_lock) ||     \
> +                             cgroup_lock_is_held() || (__c))
> +
> +static inline struct cgroup_subsys_state *
> +task_subsys_state(struct task_struct *task, int subsys_id)
>  {
> -       return rcu_dereference_check(task->cgroups->subsys[subsys_id],
> -                                    rcu_read_lock_held() ||
> -                                    lockdep_is_held(&task->alloc_lock) ||
> -                                    cgroup_lock_is_held());
> +       return task_subsys_state_check(task, subsys_id, false);
>  }
>
>  static inline struct cgroup* task_cgroup(struct task_struct *task,
> diff --git a/kernel/sched.c b/kernel/sched.c
> index f8b8996..e01bb45 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -306,32 +306,26 @@ static int init_task_group_load = INIT_TASK_GROUP_LOAD;
>  */
>  struct task_group init_task_group;
>
> -/* return group to which a task belongs */
> +/*
> + * Return the group to which this tasks belongs.
> + *
> + * We use task_subsys_state_check() and extend the RCU verification
> + * with lockdep_is_held(&task_rq(p)->lock) because cpu_cgroup_attach()
> + * holds that lock for each task it moves into the cgroup. Therefore
> + * by holding that lock, we pin the task to the current cgroup.
> + */
>  static inline struct task_group *task_group(struct task_struct *p)
>  {
> -       struct task_group *tg;
> +       struct cgroup_subsys_state *css;
>
> -#ifdef CONFIG_CGROUP_SCHED
> -       tg = container_of(task_subsys_state(p, cpu_cgroup_subsys_id),
> -                               struct task_group, css);
> -#else
> -       tg = &init_task_group;
> -#endif
> -       return tg;
> +       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
> +                       lockdep_is_held(&task_rq(p)->lock));
> +       return container_of(css, struct task_group, css);
>  }
>
>  /* Change a task's cfs_rq and parent entity if it moves across CPUs/groups */
>  static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
>  {
> -       /*
> -        * Strictly speaking this rcu_read_lock() is not needed since the
> -        * task_group is tied to the cgroup, which in turn can never go away
> -        * as long as there are tasks attached to it.
> -        *
> -        * However since task_group() uses task_subsys_state() which is an
> -        * rcu_dereference() user, this quiets CONFIG_PROVE_RCU.
> -        */
> -       rcu_read_lock();
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>        p->se.cfs_rq = task_group(p)->cfs_rq[cpu];
>        p->se.parent = task_group(p)->se[cpu];
> @@ -341,7 +335,6 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
>        p->rt.rt_rq  = task_group(p)->rt_rq[cpu];
>        p->rt.parent = task_group(p)->rt_se[cpu];
>  #endif
> -       rcu_read_unlock();
>  }
>
>  #else
> @@ -4465,16 +4458,6 @@ recheck:
>        }
>
>        if (user) {
> -#ifdef CONFIG_RT_GROUP_SCHED
> -               /*
> -                * Do not allow realtime tasks into groups that have no runtime
> -                * assigned.
> -                */
> -               if (rt_bandwidth_enabled() && rt_policy(policy) &&
> -                               task_group(p)->rt_bandwidth.rt_runtime == 0)
> -                       return -EPERM;
> -#endif
> -
>                retval = security_task_setscheduler(p, policy, param);
>                if (retval)
>                        return retval;
> @@ -4490,6 +4473,26 @@ recheck:
>         * runqueue lock must be held.
>         */
>        rq = __task_rq_lock(p);
> +
> +       retval = 0;
> +#ifdef CONFIG_RT_GROUP_SCHED
> +       if (user) {
> +               /*
> +                * Do not allow realtime tasks into groups that have no runtime
> +                * assigned.
> +                */
> +               if (rt_bandwidth_enabled() && rt_policy(policy) &&
> +                               task_group(p)->rt_bandwidth.rt_runtime == 0)
> +                       retval = -EPERM;
> +
> +               if (retval) {
> +                       __task_rq_unlock(rq);
> +                       raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +                       return retval;
> +               }
> +       }
> +#endif
> +
>        /* recheck policy now with rq lock held */
>        if (unlikely(oldpolicy != -1 && oldpolicy != p->policy)) {
>                policy = oldpolicy = -1;
>
>

  CC      kernel/sched.o
kernel/sched.c: In function ‘task_group’:
kernel/sched.c:321: error: implicit declaration of function ‘task_rq’
kernel/sched.c:321: error: invalid type argument of ‘->’ (have ‘int’)
make[1]: *** [kernel/sched.o] Error 1

I had to apply with fuzz.  Did it mess up?

static inline struct task_group *task_group(struct task_struct *p)
{
       struct cgroup_subsys_state *css;

       css = task_subsys_state_check(p, cpu_cgroup_subsys_id,
                       lockdep_is_held(&task_rq(p)->lock));
       return container_of(css, struct task_group, css);
}
--
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