netdev - Re: [PATCH v2 9/9] netfilter: Replace smp_read_barrier_depends() with lockless

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Date:	Fri, 21 Nov 2014 16:57:00 -0500
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Pablo Neira Ayuso <pablo@...filter.org>,
	Patrick McHardy <kaber@...sh.net>,
	Jozsef Kadlecsik <kadlec@...ckhole.kfki.hu>,
	"David S. Miller" <davem@...emloft.net>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	netfilter-devel@...r.kernel.org, coreteam@...filter.org,
	"open list:NETWORKING [IPv4/..." <netdev@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 9/9] netfilter: Replace smp_read_barrier_depends() with lockless_dereference()

On Fri, Nov 21, 2014 at 11:12 AM, Eric Dumazet <eric.dumazet@...il.com> 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 <bobby.prani@...il.com>
>> ---
>>  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
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