[MPTCP] Re: [PATCH mptcp-next 1/9] mptcp: add skeleton to sync msk socket options to subflows

[Paolo Abeni <pabeni@redhat.com>]

On Fri, 2021-03-19 at 13:34 +0100, Florian Westphal wrote:
> Paolo Abeni <pabeni@redhat.com> wrote:
> > There are a few __mptcp_flush_join_list() call-sites which are already
> > in process context, e.g.:
> > 
> > mptcp_stream_accept()
> > __mptcp_push_pending()
> > __mptcp_move_skbs()
> > mptcp_disconnect()
> > 
> > what renaming mptcp_work_flush_join_list() in mptcp_flush_join_list() -
> > or mptcp_work_flush_join_list_lock() or some better name - and use it
> > in the above places?
> 
> What would be the advanage?

Uhmm... the need for synchronization should be quite a rare event,
right? I guess we can affort calling the workqueue on such occasion,
but it's a bit strange to me schedule the workqueue to get process
context when alredy in process context.

BTW there a few scenarios which are not 110% clear to me:

1)
listen(s0)
s1 = accept() // main msk

setsockopt(s1, a)

[2nd subflow is accepted]
// s1->setsockopt_seq == 1, s1->setsockopt_seq _old == 0

mptcp_sockopt_sync_all()
// s1->setsockopt_seq _old == 1

[3rd subflow is accepted]
// s1->setsockopt_seq == 1, s1->setsockopt_seq _old == 1, no
synchronzation ?!?
// but the 3rd subflow will inherit it's sockopt from s0, so
// it should need that?

2)
listen(s0)
s1 = accept() // main msk

setsockopt(s0, a)

[2nd subflow is accepted, after the above sockopt]
// s1->setsockopt_seq == 0, s1->setsockopt_seq _old == 0, no
// synchronzation ?!?
// but the 2nd subflow have different options inherited by s0?!?

3)
subflow generated by port-based endpoint

I think we need some additional handling for the above ?!? e.g.
fetching setsockopt_seq_old from the listener sockets into the subflow
at syn_recv time and in _flush_join_list do/schedule the
synchronization if at least a subflow has setsockopt_seq != msk-
>setsockopt_seq.

please let me know if the above makes any sense ;)

/P
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