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