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

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Date:   Wed, 20 Dec 2017 15:05:11 -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 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. 

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.