From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexei Starovoitov Subject: Re: [PATCH v8 04/11] net/mlx4_en: add support for fast rx drop bpf program Date: Mon, 18 Jul 2016 19:45:10 -0700 Message-ID: <20160719024509.GB49474@ast-mbp.thefacebook.com> References: <1468309894-26258-5-git-send-email-bblanco@plumgrid.com> <20160714092543.776f8d8c@redhat.com> <20160715033057.GA98180@ast-mbp.thefacebook.com> <20160715164744.GA3693@ast-mbp.thefacebook.com> <20160718091023.GA29198@pox.localdomain> <20160718124811.GB2979@pox.localdomain> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Thomas Graf , Jesper Dangaard Brouer , Brenden Blanco , "David S. Miller" , Linux Kernel Network Developers , Jamal Hadi Salim , Saeed Mahameed , Martin KaFai Lau , Ari Saha , Or Gerlitz , john fastabend , Hannes Frederic Sowa , Daniel Borkmann To: Tom Herbert Return-path: Received: from mail-pf0-f180.google.com ([209.85.192.180]:35717 "EHLO mail-pf0-f180.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752452AbcGSCpQ (ORCPT ); Mon, 18 Jul 2016 22:45:16 -0400 Received: by mail-pf0-f180.google.com with SMTP id x72so2135625pfd.2 for ; Mon, 18 Jul 2016 19:45:16 -0700 (PDT) Content-Disposition: inline In-Reply-To: Sender: netdev-owner@vger.kernel.org List-ID: On Mon, Jul 18, 2016 at 03:07:01PM +0200, Tom Herbert wrote: > On Mon, Jul 18, 2016 at 2:48 PM, Thomas Graf wrote: > > On 07/18/16 at 01:39pm, Tom Herbert wrote: > >> On Mon, Jul 18, 2016 at 11:10 AM, Thomas Graf wrote: > >> > I agree with that but I would like to keep the current per net_device > >> > atomic properties. > >> > >> I don't see that see that there is any synchronization guarantees > >> using xchg. For instance, if the pointer is set right after being read > >> by a thread for one queue and right before being read by a thread for > >> another queue, this could result in the old and new program running > >> concurrently or old one running after new. If we need to synchronize > >> the operation across all queues then sequence > >> ifdown,modify-config,ifup will work. > > > > Right, there are no synchronization guarantees between threads and I > > don't think that's needed. The guarantee that is provided is that if > > I replace a BPF program, the replace either succeeds in which case > > all packets have been either processed by the old or new program. Or > > the replace failed in which case the old program was left intact and > > all packets are still going through the old program. > > > > This is a nice atomic replacement principle which would be nice to > > preserve. > > Sure, if replace operation fails then old program should remain in > place. But xchg can't fail, so it seems like part is just giving a > false sense of security that program replacement is somehow > synchronized across queues. good point. we do read_once at the beginning of napi, so we can process a bunch of packets in other cpus even after xchg is all done. Then I guess we can have a prog pointers in rings and it only marginally increases the race. Why not if it doesn't increase the patch complexity... btw we definitely want to avoid drain/start/stop or any slow operation during prog xchg. When xdp is used for dos, the prog swap needs to be fast.