From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paolo Abeni Subject: Re: [PATCH net-next 2/3] udp: avoid a cache miss on dequeue Date: Thu, 01 Jun 2017 18:21:53 +0200 Message-ID: <1496334113.9312.8.camel@redhat.com> References: <1496250043.27480.6.camel@edumazet-glaptop3.roam.corp.google.com> <1496313592.4872.14.camel@redhat.com> <1496332699.27480.14.camel@edumazet-glaptop3.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit Cc: netdev@vger.kernel.org, "David S. Miller" , Eric Dumazet To: Eric Dumazet Return-path: Received: from mx1.redhat.com ([209.132.183.28]:54394 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752117AbdFAQVz (ORCPT ); Thu, 1 Jun 2017 12:21:55 -0400 In-Reply-To: <1496332699.27480.14.camel@edumazet-glaptop3.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: On Thu, 2017-06-01 at 08:58 -0700, Eric Dumazet wrote: > On Thu, 2017-06-01 at 12:39 +0200, Paolo Abeni wrote: > > On Wed, 2017-05-31 at 10:00 -0700, Eric Dumazet wrote: > > > On Mon, 2017-05-29 at 17:27 +0200, Paolo Abeni wrote: > > > > Since UDP no more uses sk->destructor, we can clear completely > > > > the skb head state before enqueuing. > > > > > > ... > > > > > > > @@ -1739,6 +1740,9 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb) > > > > sk_mark_napi_id_once(sk, skb); > > > > } > > > > > > > > + /* drop all pending head states; dst, nf and sk are dropped by caller */ > > > > + secpath_reset(skb); > > > > + > > > > > > I wonder if using skb_release_head_state() would be more appropriate ? > > > > > > Surely more descriptive and probably not more expensive since all > > > cache lines should be already hot at this point. > > > > Thank you for reviewing this. > > > > I would prefer not adding more code to the core, but I think we would > > need something new, like: > > > > skb_reset_head_state() > > { > > skb_dst_drop(skb); > > secpath_reset(skb); > > nf_reset(skb); > > skb_orphan(skb); > > } > > > > because elsewhere the skb could be in inconsistent state: skb->sp != > > NULL but with its refcount is already decremented. WDYT? > > I do not believe skb->sk is set anymore in UDP receive path. > > If early demux sets skb->sk for a moment, skb_steal_sock() would set > skb->sk back to NULL I'm sorry, I do not follow. I'm concerned about the secpath field (skb- >sp), which is the only one that can be not NULL in __udp_queue_rcv_skb(). If the secpath is not NULL, calling there secpath_reset() (or the to- be-introduced skb_reset_head_state()), we will properly release it and we will clear the field, too. Calling skb_release_head_state() in the same scenario, we release the secpath, but we don't clear it. So if the packet is later dropped we will get a double free, unless we add and use a specialized a free_stateless_skb(), too. Paolo