Re: [PATCH] sched/numa: fix unsafe get_task_struct() in task_numa_assign()

[Kirill Tkhai <tkhai@yandex.ru>]

18.10.2014, 01:40, "Oleg Nesterov" <oleg@redhat.com>:
> The lockless get_task_struct(tsk) is only safe if tsk == current
> and didn't pass exit_notify(), or if this tsk was found on a rcu
> protected list (say, for_each_process() or find_task_by_vpid()).
> IOW, it is only safe if release_task() was not called before we
> take rcu_read_lock(), in this case we can rely on the fact that
> delayed_put_pid() can not drop the (potentially) last reference
> until rcu_read_unlock().
>
> And as Kirill pointed out task_numa_compare()->task_numa_assign()
> path does get_task_struct(dst_rq->curr) and this is not safe. The
> task_struct itself can't go away, but rcu_read_lock() can't save
> us from the final put_task_struct() in finish_task_switch(); this
> reference goes away without rcu gp.
>
> Reported-by: Kirill Tkhai <ktkhai@parallels.com>
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  kernel/sched/fair.c |    8 +++++++-
>  1 files changed, 7 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0090e8c..52049b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1158,7 +1158,13 @@ static void task_numa_compare(struct task_numa_env *env,
>
>          rcu_read_lock();
>          cur = ACCESS_ONCE(dst_rq->curr);
> - if (cur->pid == 0) /* idle */
> + /*
> + * No need to move the exiting task, and this ensures that ->curr
> + * wasn't reaped and thus get_task_struct() in task_numa_assign()
> + * is safe; note that rcu_read_lock() can't protect from the final
> + * put_task_struct() after the last schedule().
> + */
> + if (is_idle_task(cur) || (cur->flags & PF_EXITING))
>                  cur = NULL;
>
>          /*

Oleg, I've looked once again, and now it's not good for me.
Where is the guarantee this memory hasn't been allocated again?
If so, PF_EXITING is not of the task we are interesting, but it's
not a task's even.

rcu_read_lock()                   ...                           ...
cur = ACCESS_ONCE(dst_rq->curr);  ...                           ...
<interrupt>                       rq->curr = next;              ...
<interrupt>                           put_prev_task()           ...
<interrupt>                               __put_prev_task       ...
<interrupt>                                  kmem_cache_free()  ...
<interrupt>                                  ...                <alocated again>
<interrupt>                                  ...                memset(, 0, )
<interrupt>                                  ...                ...
if (cur->flags & PF_EXITING)                 ...                ...
    <no>                                     ...                ...
get_task_struct()                            ...                ...

Kirill