Re: WARNING: ODEBUG bug in tcindex_destroy_work (3)

[Paul E. McKenney]

On Wed, Mar 25, 2020 at 11:36:16AM -0700, Cong Wang wrote:
> On Mon, Mar 23, 2020 at 6:01 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >
> > Cong Wang <xiyou.wangcong@xxxxxxxxx> writes:
> > > On Mon, Mar 23, 2020 at 2:14 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> 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.

And the workqueues might well guarantee the ordering in cases where the
pair of RCU callbacks are invoked in a known order.  But that workqueues
ordering guarantee does not extend upstream to RCU, nor do I know of a
reasonable way to make this happen within the confines of RCU.

If you have ideas, please do not keep them a secret, but please also
understand that call_rcu() must meet some pretty severe performance and
scalability constraints.

I suppose that queue_rcu_work() could track outstanding call_rcu()
invocations, and (one way or another) defer the second queue_rcu_work()
if a first one is still pending from the current task, but that might not
make the common-case user of queue_rcu_work() all that happy.  But perhaps
there is a way to restrict these semantics to ordered workqueues.  In that
case, one could imagine the second and subsequent too-quick call to
queue_rcu_work() using the rcu_head structure's ->next field to queue these
too-quick callbacks, and then having rcu_work_rcufn() check for queued
too-quick callbacks, queuing the first one.

But I must defer to Tejun on this one.

And one additional caution...  This would meter out ordered
queue_rcu_work() requests at a rate of no faster than one per RCU
grace period.  The queue might build up, resulting in long delays.
Are you sure that your use case can live with this?

> > > 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.

But don't you also have a situation where there might be a large group
of queue_rcu_work() invocations whose order doesn't matter, followed by a
single queue_rcu_work() invocation that must be ordered after the earlier
group?  If so, ordering -all- of these invocations might be overkill.

Or did I misread your code?

							Thanx, Paul