Re: WARNING: ODEBUG bug in tcindex_destroy_work (3)

[Cong Wang <xiyou.wangcong@gmail.com>]

On Mon, Mar 23, 2020 at 6:01 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Cong Wang <xiyou.wangcong@gmail.com> writes:
> > On Mon, Mar 23, 2020 at 2:14 PM Thomas Gleixner <tglx@linutronix.de> wrote:
> >> > We use an ordered workqueue for tc filters, so these two
> >> > works are executed in the same order as they are queued.
> >>
> >> The workqueue is ordered, but look how the work is queued on the work
> >> queue:
> >>
> >> tcf_queue_work()
> >>   queue_rcu_work()
> >>     call_rcu(&rwork->rcu, rcu_work_rcufn);
> >>
> >> So after the grace period elapses rcu_work_rcufn() queues it in the
> >> actual work queue.
> >>
> >> Now tcindex_destroy() is invoked via tcf_proto_destroy() which can be
> >> invoked from preemtible context. Now assume the following:
> >>
> >> CPU0
> >>   tcf_queue_work()
> >>     tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);
> >>
> >> -> Migration
> >>
> >> CPU1
> >>    tcf_queue_work(&p->rwork, tcindex_destroy_work);
> >>
> >> So your RCU callbacks can be placed on different CPUs which obviously
> >> has no ordering guarantee at all. See also:
> >
> > Good catch!
> >
> > I thought about this when I added this ordered workqueue, but it
> > seems I misinterpret max_active, so despite we have max_active==1,
> > more than 1 work could still be queued on different CPU's here.
>
> The workqueue is not the problem. it works perfectly fine. The way how
> the work gets queued is the issue.

Well, a RCU work is also a work, so the ordered workqueue should
apply to RCU works too, from users' perspective. Users should not
need to learn queue_rcu_work() is actually a call_rcu() which does
not guarantee the ordering for an ordered workqueue.


> > I don't know how to fix this properly, I think essentially RCU work
> > should be guaranteed the same ordering with regular work. But this
> > seems impossible unless RCU offers some API to achieve that.
>
> I don't think that's possible w/o putting constraints on the flexibility
> of RCU (Paul of course might disagree).
>
> I assume that the filters which hang of tcindex_data::perfect and
> tcindex_data:p must be freed before tcindex_data, right?
>
> Refcounting of tcindex_data should do the trick. I.e. any element which
> you add to a tcindex_data instance takes a refcount and when that is
> destroyed then the rcu/work callback drops a reference which once it
> reaches 0 triggers tcindex_data to be freed.

Yeah, but the problem is more than just tcindex filter, we have many
places make the same assumption of ordering.

Thanks!