From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-15.5 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 385DEC48BDF for ; Fri, 18 Jun 2021 23:27:33 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1952F61164 for ; Fri, 18 Jun 2021 23:27:33 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S234322AbhFRX3m (ORCPT ); Fri, 18 Jun 2021 19:29:42 -0400 Received: from www62.your-server.de ([213.133.104.62]:58866 "EHLO www62.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230024AbhFRX3j (ORCPT ); Fri, 18 Jun 2021 19:29:39 -0400 Received: from sslproxy02.your-server.de ([78.47.166.47]) by www62.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92.3) (envelope-from ) id 1luNtY-0006gO-BZ; Sat, 19 Jun 2021 01:27:28 +0200 Received: from [85.7.101.30] (helo=linux.home) by sslproxy02.your-server.de with esmtpsa (TLSv1.3:TLS_AES_256_GCM_SHA384:256) (Exim 4.92) (envelope-from ) id 1luNtY-00037V-4T; Sat, 19 Jun 2021 01:27:28 +0200 Subject: Re: [PATCH bpf-next v3 03/16] xdp: add proper __rcu annotations to redirect map entries To: =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= , bpf@vger.kernel.org, netdev@vger.kernel.org Cc: Martin KaFai Lau , Hangbin Liu , Jesper Dangaard Brouer , Magnus Karlsson , "Paul E . McKenney" , Jakub Kicinski References: <20210617212748.32456-1-toke@redhat.com> <20210617212748.32456-4-toke@redhat.com> From: Daniel Borkmann Message-ID: <1881ecbe-06ec-6b0a-836c-033c31fabef4@iogearbox.net> Date: Sat, 19 Jun 2021 01:27:27 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.2 MIME-Version: 1.0 In-Reply-To: <20210617212748.32456-4-toke@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Authenticated-Sender: daniel@iogearbox.net X-Virus-Scanned: Clear (ClamAV 0.103.2/26205/Fri Jun 18 13:18:00 2021) Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On 6/17/21 11:27 PM, Toke Høiland-Jørgensen wrote: > XDP_REDIRECT works by a three-step process: the bpf_redirect() and > bpf_redirect_map() helpers will lookup the target of the redirect and store > it (along with some other metadata) in a per-CPU struct bpf_redirect_info. > Next, when the program returns the XDP_REDIRECT return code, the driver > will call xdp_do_redirect() which will use the information thus stored to > actually enqueue the frame into a bulk queue structure (that differs > slightly by map type, but shares the same principle). Finally, before > exiting its NAPI poll loop, the driver will call xdp_do_flush(), which will > flush all the different bulk queues, thus completing the redirect. > > Pointers to the map entries will be kept around for this whole sequence of > steps, protected by RCU. However, there is no top-level rcu_read_lock() in > the core code; instead drivers add their own rcu_read_lock() around the XDP > portions of the code, but somewhat inconsistently as Martin discovered[0]. > However, things still work because everything happens inside a single NAPI > poll sequence, which means it's between a pair of calls to > local_bh_disable()/local_bh_enable(). So Paul suggested[1] that we could > document this intention by using rcu_dereference_check() with > rcu_read_lock_bh_held() as a second parameter, thus allowing sparse and > lockdep to verify that everything is done correctly. > > This patch does just that: we add an __rcu annotation to the map entry > pointers and remove the various comments explaining the NAPI poll assurance > strewn through devmap.c in favour of a longer explanation in filter.c. The > goal is to have one coherent documentation of the entire flow, and rely on > the RCU annotations as a "standard" way of communicating the flow in the > map code (which can additionally be understood by sparse and lockdep). > > The RCU annotation replacements result in a fairly straight-forward > replacement where READ_ONCE() becomes rcu_dereference_check(), WRITE_ONCE() > becomes rcu_assign_pointer() and xchg() and cmpxchg() gets wrapped in the > proper constructs to cast the pointer back and forth between __rcu and > __kernel address space (for the benefit of sparse). The one complication is > that xskmap has a few constructions where double-pointers are passed back > and forth; these simply all gain __rcu annotations, and only the final > reference/dereference to the inner-most pointer gets changed. > > With this, everything can be run through sparse without eliciting > complaints, and lockdep can verify correctness even without the use of > rcu_read_lock() in the drivers. Subsequent patches will clean these up from > the drivers. > > [0] https://lore.kernel.org/bpf/20210415173551.7ma4slcbqeyiba2r@kafai-mbp.dhcp.thefacebook.com/ > [1] https://lore.kernel.org/bpf/20210419165837.GA975577@paulmck-ThinkPad-P17-Gen-1/ > > Signed-off-by: Toke Høiland-Jørgensen > --- > include/net/xdp_sock.h | 2 +- > kernel/bpf/cpumap.c | 13 +++++++---- > kernel/bpf/devmap.c | 49 ++++++++++++++++++------------------------ > net/core/filter.c | 28 ++++++++++++++++++++++++ > net/xdp/xsk.c | 4 ++-- > net/xdp/xsk.h | 4 ++-- > net/xdp/xskmap.c | 29 ++++++++++++++----------- > 7 files changed, 80 insertions(+), 49 deletions(-) [...] > __dev_map_entry_free); > diff --git a/net/core/filter.c b/net/core/filter.c > index caa88955562e..0b7db5c70385 100644 > --- a/net/core/filter.c > +++ b/net/core/filter.c > @@ -3922,6 +3922,34 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = { > .arg2_type = ARG_ANYTHING, > }; > > +/* XDP_REDIRECT works by a three-step process, implemented in the functions > + * below: > + * > + * 1. The bpf_redirect() and bpf_redirect_map() helpers will lookup the target > + * of the redirect and store it (along with some other metadata) in a per-CPU > + * struct bpf_redirect_info. > + * > + * 2. When the program returns the XDP_REDIRECT return code, the driver will > + * call xdp_do_redirect() which will use the information in struct > + * bpf_redirect_info to actually enqueue the frame into a map type-specific > + * bulk queue structure. > + * > + * 3. Before exiting its NAPI poll loop, the driver will call xdp_do_flush(), > + * which will flush all the different bulk queues, thus completing the > + * redirect. > + * > + * Pointers to the map entries will be kept around for this whole sequence of > + * steps, protected by RCU. However, there is no top-level rcu_read_lock() in > + * the core code; instead, the RCU protection relies on everything happening > + * inside a single NAPI poll sequence, which means it's between a pair of calls > + * to local_bh_disable()/local_bh_enable(). > + * > + * The map entries are marked as __rcu and the map code makes sure to > + * dereference those pointers with rcu_dereference_check() in a way that works > + * for both sections that to hold an rcu_read_lock() and sections that are > + * called from NAPI without a separate rcu_read_lock(). The code below does not > + * use RCU annotations, but relies on those in the map code. One more follow-up question related to tc BPF: given we do use rcu_read_lock_bh() in case of sch_handle_egress(), could we also remove the rcu_read_lock() pair from cls_bpf_classify() then? It would also be great if this scenario in general could be placed under the Documentation/RCU/whatisRCU.rst as an example, so we could refer to the official doc on this, too, if Paul is good with this. Could you also update the RCU comment in bpf_prog_run_xdp()? Or alternatively move all the below driver comments in there as a single location? /* This code is invoked within a single NAPI poll cycle and thus under * local_bh_disable(), which provides the needed RCU protection. */ Thanks, Daniel