Messages in this thread | | | Date | Mon, 23 Mar 2020 19:05:04 -0700 | From | "Paul E. McKenney" <> | Subject | Re: WARNING: ODEBUG bug in tcindex_destroy_work (3) |
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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.
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.
Thanx, Paul
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