lkml.org 
[lkml]   [2014]   [Nov]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [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@gmail.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@gmail.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


\
 
 \ /
  Last update: 2014-11-21 23:41    [W:0.129 / U:4.232 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site