On Wed, Dec 20, 2017 at 3:05 PM, John Fastabend
<john.fastab...@gmail.com> wrote:
> On 12/20/2017 02:41 PM, Cong Wang wrote:
>> On Wed, Dec 20, 2017 at 12:09 PM, John Fastabend
>> <john.fastab...@gmail.com> wrote:
>>> RCU grace period is needed for lockless qdiscs added in the commit
>>> c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array").
>>>
>>> It is needed now that qdiscs may be lockless otherwise we risk
>>> free'ing a qdisc that is still in use from datapath. Additionally,
>>> push list cleanup into RCU callback. Otherwise we risk the datapath
>>> adding skbs during removal.
>>
>> What about qdisc_graft() -> dev_deactivate() -> synchronize_net() ?
>> It doesn't work with your "lockless" patches?
>>
>
> Well this is only in the 'parent == NULL' case otherwise we call
> cops->graft(). Most sch_* seem to use qdisc_replace and this uses
> sch_tree_lock().
>
> The only converted qdisc mq and mqprio at this point don't care
> though and do their own dev_deactivate/activate. So its not fixing
> anything in the above mentioned commit.Sure, removing a class does not impact the whole device, but removing the root qdisc does. After your "lockless", skb_array_consume_bh() is called in pfifo_fast_reset() and ptr_ring_cleanup() is called in pfifo_fast_destroy(), assuming skb_array is not buggy, what race do we have here with datapath? > > I still think it will need to be done eventually. If it resolves > the miniq case it seems like a good idea. Although per Jakub's comment > perhaps I pulled too much into the RCU handler. The case Jakub reported is a RCU callback missing a rcu barrier. I don't understand why you keep believing it is RCU readers on datapath. Not even to mention ingress is not affected by your "lockless" thing.
