Re: Panic at tcp_xmit_retransmit_queue

[sbs <gexlie@gmail.com>]

thank you very much, have stable running server for a week and it
seems that it works like a charm now, i havent detected any panics
since i apply the patch. although seems that the problem stops
ocurring cause i dont see any debug information through netconsole

On Mon, Feb 15, 2010 at 4:21 PM, Ilpo Järvinen
<ilpo.jarvinen@helsinki.fi> wrote:
> On Wed, 3 Feb 2010, Ilpo Järvinen wrote:
>
>> On Mon, 1 Feb 2010, sbs wrote:
>>
>> > actually removing netconsole from kernel didnt help.
>> > i found many guys with the same problem but with different hardware
>> > configurations here:
>> >
>> > freez in TCP stack :
>> > http://bugzilla.kernel.org/show_bug.cgi?id=14470
>> >
>> > is there someone who can investigate it?
>> >
>> >
>> > On Tue, Jan 19, 2010 at 7:13 PM, sbs <gexlie@gmail.com> wrote:
>> > > We are hiting kernel panics on servers with nVidia MCP55 NICs once a day;
>> > > it appears usualy under a high network trafic ( around 10000Mbit/s) but
>> > > it is not a rule, it has happened even on low trafic.
>> > >
>> > > Servers are used as nginx+static content
>> > > On 2 equal servers this panic happens aprox 2 times a day depending on
>> > > network load. Machine completly freezes till the netconsole reboots.
>> > >
>> > > Kernel: 2.6.32.3
>> > >
>> > > what can it be? whats wrong with tcp_xmit_retransmit_queue() function ?
>> > > can anyone explain or fix?
>>
>> You might want to try with to debug patch below. It might even make the
>> box to survive the event (if I got it coded right).
>
> Here should be a better version of the debug patch, hopefully the infinite
> looping is now gone.
>
> --
>  i.
>
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 383ce23..4672a30 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2186,6 +2186,42 @@ static int tcp_can_forward_retransmit(struct sock *sk)
>        return 1;
>  }
>
> +static void print_queue(struct sock *sk, struct sk_buff *old, struct sk_buff *hole)
> +{
> +       struct tcp_sock *tp = tcp_sk(sk);
> +       struct sk_buff *skb, *prev;
> +
> +       skb = tcp_write_queue_head(sk);
> +       prev = (struct sk_buff *)(&sk->sk_write_queue);
> +
> +       if (skb == NULL) {
> +               printk("NULL head, pkts %u\n", tp->packets_out);
> +               return;
> +       }
> +       printk("head %p tail %p sendhead %p oldhint %p now %p hole %p high %u\n",
> +              tcp_write_queue_head(sk), tcp_write_queue_tail(sk),
> +              tcp_send_head(sk), old, tp->retransmit_skb_hint, hole,
> +              tp->retransmit_high);
> +
> +       while (skb) {
> +               printk("skb %p (%u-%u) next %p prev %p sacked %u\n",
> +                      skb, TCP_SKB_CB(skb)->seq, TCP_SKB_CB(skb)->end_seq,
> +                      skb->next, skb->prev, TCP_SKB_CB(skb)->sacked);
> +               if (prev != skb->prev)
> +                       printk("Inconsistent prev\n");
> +
> +               if (skb == tcp_write_queue_tail(sk)) {
> +                       if (skb->next != (struct sk_buff *)(&sk->sk_write_queue))
> +                               printk("Improper next at tail\n");
> +                       return;
> +               }
> +
> +               prev = skb;
> +               skb = skb->next;
> +       }
> +       printk("Encountered unexpected NULL\n");
> +}
> +
>  /* This gets called after a retransmit timeout, and the initially
>  * retransmitted data is acknowledged.  It tries to continue
>  * resending the rest of the retransmit queue, until either
> @@ -2194,12 +2230,15 @@ static int tcp_can_forward_retransmit(struct sock *sk)
>  * based retransmit packet might feed us FACK information again.
>  * If so, we use it to avoid unnecessarily retransmissions.
>  */
> +static int caught_it = 0;
> +
>  void tcp_xmit_retransmit_queue(struct sock *sk)
>  {
>        const struct inet_connection_sock *icsk = inet_csk(sk);
>        struct tcp_sock *tp = tcp_sk(sk);
>        struct sk_buff *skb;
>        struct sk_buff *hole = NULL;
> +       struct sk_buff *old = tp->retransmit_skb_hint;
>        u32 last_lost;
>        int mib_idx;
>        int fwd_rexmitting = 0;
> @@ -2217,6 +2256,16 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
>                last_lost = tp->snd_una;
>        }
>
> +checknull:
> +       if (skb == NULL) {
> +               if (!caught_it)
> +                       print_queue(sk, old, hole);
> +               caught_it++;
> +               if (net_ratelimit())
> +                       printk("Errors caught so far %u\n", caught_it);
> +               return;
> +       }
> +
>        tcp_for_write_queue_from(skb, sk) {
>                __u8 sacked = TCP_SKB_CB(skb)->sacked;
>
> @@ -2257,7 +2306,7 @@ begin_fwd:
>                } else if (!(sacked & TCPCB_LOST)) {
>                        if (hole == NULL && !(sacked & (TCPCB_SACKED_RETRANS|TCPCB_SACKED_ACKED)))
>                                hole = skb;
> -                       continue;
> +                       goto cont;
>
>                } else {
>                        last_lost = TCP_SKB_CB(skb)->end_seq;
> @@ -2268,7 +2317,7 @@ begin_fwd:
>                }
>
>                if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
> -                       continue;
> +                       goto cont;
>
>                if (tcp_retransmit_skb(sk, skb))
>                        return;
> @@ -2278,6 +2327,9 @@ begin_fwd:
>                        inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
>                                                  inet_csk(sk)->icsk_rto,
>                                                  TCP_RTO_MAX);
> +cont:
> +               skb = skb->next;
> +               goto checknull;
>        }
>  }
>