netdev - Re: BUG: using smp_processor_id() in preemptible [00000000] code: avahi-daemon: caller is netif

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Date:	Thu, 15 Apr 2010 16:27:19 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, therbert@...gle.com, eparis@...hat.com,
	netdev@...r.kernel.org
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code: 
	avahi-daemon: caller is netif_rx

On Thu, Apr 15, 2010 at 3:57 PM, David Miller <davem@...emloft.net> wrote:
>
> Why?  If we are in an interrupt (either soft or hard) then
> smp_processor_id() is stable, and valid.
>
> Changli, I find it very frustrating to communicate with you, you are
> very terse in your descriptions and analysis and you make many simple
> errors that would be avoided if you spent more time thinking about
> things before sending your emails. :-/
>
> Instead of just showing some pseudo patch, state WHY it is needed.
> Talk about the execution state of environment and what rules or other
> things are being violated which must be corrected.
>

Sorry. English isn't my native language, so sometimes I can't describe
myself clearly.

I think the following patch from Eric should be applied instead.

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index c65f18e..d1bcc9f 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -120,7 +120,7 @@ static int ip_dev_loopback_xmit(struct sk_buff *newskb)
       newskb->pkt_type = PACKET_LOOPBACK;
       newskb->ip_summed = CHECKSUM_UNNECESSARY;
       WARN_ON(!skb_dst(newskb));
-       netif_rx(newskb);
+       netif_rx_ni(newskb);
       return 0;
 }

As you know  "netif_rx() must be invoked from a hardware or software
interrupt, which implies preemption disabled.", obviously
ip_dev_loopback_xmit() doesn't obey this rule, so the crash isn't the
fault of net_rx(). If there are other users, who don't obey this rule,
they should be fixed too.

For this patch:

-       cpu = smp_processor_id();
+       ret = enqueue_to_backlog(skb, get_cpu());
+       put_cpu();

You said: " If we are in an interrupt (either soft or hard) then
smp_processor_id() is stable, and valid.". so we don't need to call
get_cpu() instead of smp_processor_id(). get_cpu() brings no good but
additional cost preempt_disable().

-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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