netdev - Re: [PATCH] net: Revert "net_sched: no need to free qdisc in RCU callback"

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Date:   Wed, 20 Dec 2017 15:34:05 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Jiri Pirko <jiri@...nulli.us>, David Miller <davem@...emloft.net>,
        Jakub Kicinski <kubakici@...pl>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH] net: Revert "net_sched: no need to free qdisc in RCU
 callback"

On 12/20/2017 03:23 PM, Cong Wang wrote:
> On Wed, Dec 20, 2017 at 3:05 PM, John Fastabend
> <john.fastabend@...il.com> wrote:
>> On 12/20/2017 02:41 PM, Cong Wang wrote:
>>> On Wed, Dec 20, 2017 at 12:09 PM, John Fastabend
>>> <john.fastabend@...il.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?
> 

None at the moment.

> 
>>
>> 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.
> 

I was thinking about the case where we want a lockless qdisc
with classes. Doing the qdisc destroy after a grace period would
solve this. Also we could start to cleanup a lot of the locking
and extra bits around 'running' qdisc and such by doing a clean
xchg on the qdisc layer. It seems that a dev_activate/deactivate
just to install a new qdisc is not needed.

Anyways future work. However if it resolves the miniq issue, as
Jiri indicated, seems like a clean fix. Although Jakub's issue
with the patch would need to be addressed. Seems he gets a WARN_ON
if the offload is not disabled but the device is unitialized.

.John