Messages in this thread | | | From | Pranith Kumar <> | Date | Fri, 21 Nov 2014 16:57:00 -0500 | Subject | Re: [PATCH v2 9/9] netfilter: Replace smp_read_barrier_depends() with lockless_dereference() |
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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
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