Messages in this thread | | | Date | Wed, 25 Mar 2020 11:58:15 -0700 | From | "Paul E. McKenney" <> | Subject | Re: WARNING: ODEBUG bug in tcindex_destroy_work (3) |
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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@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.
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
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