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
[off-list ref] wrote:

quoted

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.