Messages in this thread | | | From | Cong Wang <> | Date | Wed, 25 Mar 2020 11:53:51 -0700 | Subject | Re: WARNING: ODEBUG bug in tcindex_destroy_work (3) |
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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.
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