From: Eric Dumazet <eduma...@google.com> Date: Tue, 20 Nov 2018 10:08:12 -0800
> On Tue, Nov 20, 2018 at 10:01 AM David Miller <da...@davemloft.net> wrote: >> >> From: Eric Dumazet <eduma...@google.com> >> Date: Mon, 19 Nov 2018 17:45:55 -0800 >> >> > Under stress, softirq rx handler often hits a socket owned by the user, >> > and has to queue the packet into socket backlog. >> > >> > When this happens, skb dst refcount is taken before we escape rcu >> > protected region. This is done from __sk_add_backlog() calling >> > skb_dst_force(). >> > >> > Consumer will have to perform the opposite costly operation. >> > >> > AFAIK nothing in tcp stack requests the dst after skb was stored >> > in the backlog. If this was the case, we would have had failures >> > already since skb_dst_force() can end up clearing skb dst anyway. >> > >> > Signed-off-by: Eric Dumazet <eduma...@google.com> >> >> Hmmm, it seems to be used by connection completion code to setup the >> socket cached rx dst, right? >> >> For example tcp_finish_connect() --> icsk->icsk_af_ops->sk_rx_dst_set(sk, >> skb) > > We already cope with skb->dst being NULL there I believe. > > For reference look at > > commit 5037e9ef9454917b047f9f3a19b4dd179fbf7cd4 net: fix IP early demux > races Well, I'm sure we "handle" it. But I was more asking about the performance tradeoff, which probably is on the side of your change but I wanted to just be sure.
