netdev - Re: [PATCH net] netfilter: flowtable: Add pending bit for offload work

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Date:   Mon, 11 May 2020 13:59:39 +0200
From:   Pablo Neira Ayuso <pablo@...filter.org>
To:     Paul Blakey <paulb@...lanox.com>
Cc:     Oz Shlomo <ozsh@...lanox.com>, Roi Dayan <roid@...lanox.com>,
        netdev@...r.kernel.org, Saeed Mahameed <saeedm@...lanox.com>,
        netfilter-devel@...r.kernel.org
Subject: Re: [PATCH net] netfilter: flowtable: Add pending bit for offload
 work

On Mon, May 11, 2020 at 11:32:36AM +0300, Paul Blakey wrote:
> On 5/11/2020 1:14 AM, Pablo Neira Ayuso wrote:
[...]
> >> @@ -831,9 +832,14 @@ static void flow_offload_queue_work(struct flow_offload_work *offload)
> >>  {
> >>  	struct flow_offload_work *offload;
> >>  
> >> +	if (test_and_set_bit(NF_FLOW_HW_PENDING, &flow->flags))
> >> +		return NULL;
> > In case of stats, it's fine to lose work.
> >
> > But how does this work for the deletion case? Does this falls back to
> > the timeout deletion?
> 
> We get to nf_flow_table_offload_del (delete) in these cases:
> 
> >-------if (nf_flow_has_expired(flow) || nf_ct_is_dying(flow->ct) ||
> >-------    test_bit(NF_FLOW_TEARDOWN, &flow->flags) {
> >------->-------   ....
> >------->-------    nf_flow_offload_del(flow_table, flow);
> 
> Which are all persistent once set but the nf_flow_has_expired(flow). So we will
> try the delete
> again and again till pending flag is unset or the flow is 'saved' by the already
> queued stats updating the timeout.
> A pending stats update can't save the flow once it's marked for teardown or
> (flow->ct is dying), only delay it.

Thanks for explaining.

> We didn't mention flush, like in table free. I guess we need to flush the
> hardware workqueue
> of any pending stats work, then queue the deletion, and flush again:
> Adding nf_flow_table_offload_flush(flow_table), after
> cancel_delayed_work_sync(&flow_table->gc_work);

The "flush" makes sure that stats work runs before the deletion, to
ensure no races happen for in-transit work objects, right?

We might use alloc_ordered_workqueue() and let the workqueue handle
this problem?