Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752172AbaKUV5d (ORCPT ); Fri, 21 Nov 2014 16:57:33 -0500 Received: from mail-vc0-f170.google.com ([209.85.220.170]:33030 "EHLO mail-vc0-f170.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750948AbaKUV5b (ORCPT ); Fri, 21 Nov 2014 16:57:31 -0500 MIME-Version: 1.0 In-Reply-To: <1416586364.8629.104.camel@edumazet-glaptop2.roam.corp.google.com> References: <1416582363-20661-1-git-send-email-bobby.prani@gmail.com> <1416582363-20661-10-git-send-email-bobby.prani@gmail.com> <1416586364.8629.104.camel@edumazet-glaptop2.roam.corp.google.com> From: Pranith Kumar Date: Fri, 21 Nov 2014 16:57:00 -0500 Message-ID: Subject: Re: [PATCH v2 9/9] netfilter: Replace smp_read_barrier_depends() with lockless_dereference() To: Eric Dumazet Cc: Pablo Neira Ayuso , Patrick McHardy , Jozsef Kadlecsik , "David S. Miller" , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , netfilter-devel@vger.kernel.org, coreteam@netfilter.org, "open list:NETWORKING [IPv4/..." , open list Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 21, 2014 at 11:12 AM, Eric Dumazet wrote: > On Fri, 2014-11-21 at 10:06 -0500, Pranith Kumar wrote: >> Recently lockless_dereference() was added which can be used in place of >> hard-coding smp_read_barrier_depends(). The following PATCH makes the change. >> >> Signed-off-by: Pranith Kumar >> --- >> net/ipv4/netfilter/arp_tables.c | 3 +-- >> net/ipv4/netfilter/ip_tables.c | 3 +-- >> net/ipv6/netfilter/ip6_tables.c | 3 +-- >> 3 files changed, 3 insertions(+), 6 deletions(-) >> >> diff --git a/net/ipv4/netfilter/arp_tables.c b/net/ipv4/netfilter/arp_tables.c >> index f95b6f9..fc7533d 100644 >> --- a/net/ipv4/netfilter/arp_tables.c >> +++ b/net/ipv4/netfilter/arp_tables.c >> @@ -270,12 +270,11 @@ unsigned int arpt_do_table(struct sk_buff *skb, >> >> local_bh_disable(); >> addend = xt_write_recseq_begin(); >> - private = table->private; >> /* >> * Ensure we load private-> members after we've fetched the base >> * pointer. >> */ >> - smp_read_barrier_depends(); >> + private = lockless_dereference(table->private); >> table_base = private->entries[smp_processor_id()]; >> > > > Please carefully read the code, before and after your change, then > you'll see this change broke the code. > > Problem is that a bug like that can be really hard to diagnose and fix > later, so really you have to be very careful doing these mechanical > changes. > > IMO, current code+comment is better than with this > lockless_dereference() which in this particular case obfuscates the > code. more than anything. > > In this case we do have a lock (sort of), so lockless_dereference() is > quite misleading. > Hi Eric, Thanks for looking at this patch. I've been scratching my head since morning trying to find out what was so obviously wrong with this patch. Alas, I don't see what you do. Could you point it out and show me how incompetent I am, please? Thanks! -- Pranith -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/