Re: WARNING: ODEBUG bug in tcindex_destroy_work (3)

[Thomas Gleixner]

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.

> 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.

Thanks,

        tglx