Re: [PATCH net 6/6] net/sched: qdisc_destroy() old ingress and clsact Qdiscs before grafting

[Peilin Ye]

On Wed, May 10, 2023 at 04:15:59PM -0700, Jakub Kicinski wrote:
> My thinking was to make sure that dev->miniq_* pointers always point
> to one of the miniqs of the currently attached qdisc. Right now, on
> a quick look, those pointers are not initialized during initial graft,
> only when first filter is added, and may be cleared when filters are
> removed. But I don't think that's strictly required, miniq with no
> filters should be fine.

Ah, I see, thanks for explaining, I didn't think of that.  Looking at
sch_handle_ingress() BTW, currently mini Qdisc stats aren't being updated
(mini_qdisc_bstats_cpu_update()) if there's no filters, is this intended?
Should I keep this behavior by:

diff --git a/net/core/dev.c b/net/core/dev.c
index 735096d42c1d..9016481377e0 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5116,7 +5116,7 @@ sch_handle_ingress(struct sk_buff *skb, struct packet_type **pt_prev, int *ret,
         * that are not configured with an ingress qdisc will bail
         * out here.
         */
-       if (!miniq)
+       if (!miniq || !miniq->filter_list)
                return skb;

        if (*pt_prev) {

On Wed, May 10, 2023 at 04:15:59PM -0700, Jakub Kicinski wrote:
> On Wed, 10 May 2023 13:11:19 -0700 Peilin Ye wrote:
> > On Fri,  5 May 2023 17:16:10 -0700 Peilin Ye wrote:
> > >  Thread 1               A's refcnt   Thread 2
> > >   RTM_NEWQDISC (A, RTNL-locked)
> > >    qdisc_create(A)               1
> > >    qdisc_graft(A)                9
> > >
> > >   RTM_NEWTFILTER (X, RTNL-lockless)
> > >    __tcf_qdisc_find(A)          10
> > >    tcf_chain0_head_change(A)
> > >    mini_qdisc_pair_swap(A) (1st)
> > >             |
> > >             |                         RTM_NEWQDISC (B, RTNL-locked)
> > >            RCU                   2     qdisc_graft(B)
> > >             |                    1     notify_and_destroy(A)
> > >             |
> > >    tcf_block_release(A)          0    RTM_NEWTFILTER (Y, RTNL-lockless)
> > >    qdisc_destroy(A)                    tcf_chain0_head_change(B)
> > >    tcf_chain0_head_change_cb_del(A)    mini_qdisc_pair_swap(B) (2nd)
> > >    mini_qdisc_pair_swap(A) (3rd)                |
> > >            ...                                 ...
> >
> > Looking at the code, I think there is no guarantee that (1st) cannot
> > happen after (2nd), although unlikely?  Can RTNL-lockless RTM_NEWTFILTER
> > handlers get preempted?
>
> Right, we need qdisc_graft(B) to update the appropriate dev pointer
> to point to b1. With that the ordering should not matter. Probably
> using the ->attach() callback?

->attach() is later than dev_graft_qdisc().  We should get ready for
concurrent filter requests (i.e. have dev pointer pointing to b1) before
grafting (publishing) B.  Also currently qdisc_graft() doesn't call
->attach() for ingress and clsact Qdiscs.

But I see your point, thanks for the suggestion!  I'll try ->init() and
create v2.

Thanks,
Peilin Ye