Re: WARNING: ODEBUG bug in tcindex_destroy_work (3)

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

On Mon, Mar 23, 2020 at 7:05 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Mar 24, 2020 at 02:01:13AM +0100, Thomas Gleixner 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.
> >
> > > 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).
>
> It is possible, but it does not come for free.
>
> From an RCU/workqueues perspective, if I understand the scenario, you
> can do the following:
>
>         tcf_queue_work(&r->rwork, tcindex_destroy_rexts_work);
>
>         rcu_barrier(); // Wait for the RCU callback.
>         flush_work(...); // Wait for the workqueue handler.
>                          // But maybe for quite a few of them...
>
>         // All the earlier handlers have completed.
>         tcf_queue_work(&p->rwork, tcindex_destroy_work);
>
> This of course introduces overhead and latency.  Maybe that is not a
> problem at teardown time, or maybe the final tcf_queue_work() can itself
> be dumped into a workqueue in order to get it off of the critical path.

I personally agree, but nowadays NIC vendors care about tc filter
slow path performance as well. :-/


>
> However, depending on your constraints ...
>
> > 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.
>
> ... reference counts might work much better for you.
>

I need to think about how much work is needed for refcnting, given
other filters have the same assumption.

Thanks.