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=-3.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,URIBL_BLOCKED,USER_AGENT_NEOMUTT autolearn=ham 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 766ADC43387 for ; Thu, 10 Jan 2019 14:48:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 454AD2177B for ; Thu, 10 Jan 2019 14:48:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729269AbfAJOsT (ORCPT ); Thu, 10 Jan 2019 09:48:19 -0500 Received: from Chamillionaire.breakpoint.cc ([146.0.238.67]:53116 "EHLO Chamillionaire.breakpoint.cc" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728125AbfAJOsT (ORCPT ); Thu, 10 Jan 2019 09:48:19 -0500 Received: from fw by Chamillionaire.breakpoint.cc with local (Exim 4.89) (envelope-from ) id 1ghbcy-0006Lj-1R; Thu, 10 Jan 2019 15:48:12 +0100 Date: Thu, 10 Jan 2019 15:48:12 +0100 From: Florian Westphal To: Peter Zijlstra Cc: Florian Westphal , Anatol Pomozov , Dmitry Vyukov , paulmck@linux.ibm.com, LKML Subject: Re: seqcount usage in xt_replace_table() Message-ID: <20190110144812.mkbokbj2iyryj6lv@breakpoint.cc> References: <20190108223746.shuwx3ro7cgwz7hh@breakpoint.cc> <20190110124123.GA21224@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190110124123.GA21224@hirez.programming.kicks-ass.net> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Peter Zijlstra wrote: > /* > * Ensure contents of newinfo are visible before assigning to > * private. > */ > smp_wmb(); > table->private = newinfo; > > we have: > > smp_store_release(&table->private, newinfo); > > But what store does that second smp_wmb() order against? The comment: > > /* make sure all cpus see new ->private value */ > smp_wmb(); > > makes no sense what so ever, no smp_*() barrier can provide such > guarantees. IIRC I added this at the request of a reviewer of an earlier iteration of that patch. IIRC the concern was that compiler/hw could re-order smb_wmb(); table->private = newinfo; /* wait until all cpus are done with old table */ into: smb_wmb(); /* wait until all cpus are done with old table */ ... table->private = newinfo; and that (obviously) makes the wait-loop useless. > > Only alternative I see that might work is synchronize_rcu (the > > _do_table functions are called with rcu read lock held). > > > > I guess current scheme is cheaper though. > > Is performance a concern in this path? There is no comment justifying > this 'creative' stuff. We have to wait until all cpus are done with current iptables ruleset. Before this 'creative' change, this relied on get_counters synchronization. And that caused wait times of 30 seconds or more on busy systems. I have no objections swapping this with a synchronize_rcu() if that makes it easier. (synchronize_rcu might be confusing though, as we don't use rcu for table->private), and one 'has to know' that all the netfilter hooks, including the iptables eval loop, run with rcu_read_lock held).