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 [off-list ref] wrote:

On Fri, 2014-11-21 at 10:06 -0500, Pranith Kumar wrote:

quoted

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 <redacted>
---
 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