Re: oops in tcp_xmit_retransmit_queue() w/ v2.6.32.15

Ilpo Järvinen
2010-07-20T08:41:49Z

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On Mon, 19 Jul 2010, Tejun Heo wrote:

> Hello,
>
> On 07/16/2010 02:02 PM, Ilpo Järvinen wrote:
> > Besides, Tejun has also found that it's hint->next ptr which is NULL in
> > his case so this won't solve his case anyway. Tejun, can you confirm
> > whether it was retransmit_skb_hint->next being NULL on _entry time_ to
> > tcp_xmit_retransmit_queue() or later on in the loop after the updates done
> > by the loop itself to the hint (or that your testing didn't conclude
> > either)?
>
> Sorry about the delay. I was traveling last week. Unfortunately, I
> don't know whether ->next was NULL on entry or not. I hacked up the
> following ugly patch for the next test run. It should have everything
> which has come up till now + list and hint sanity checking before
> starting processing them. I'm planning on deploying it w/ crashdump
> enabled in several days. If I've missed something, please let me
> know.

One thing that complicates things further is the fact that the write queue
can change beneath us (ie., in tcp_retrans_try_collapse and tcp_fragment).
I've read them multiple times through and always found them innocent so
this might be just for-the-record type of a note (at least I hope so).

--
i.

> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index b4ed957..1c8b1e0 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -2190,6 +2190,53 @@ 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;
> + bool do_panic = false;
> +
> + skb = tcp_write_queue_head(sk);
> + prev = (struct sk_buff *)(&sk->sk_write_queue);
> +
> + if (skb == NULL) {
> + printk("XXX NULL head, pkts %u\n", tp->packets_out);
> + do_panic = true;
> + }
> +
> + printk("XXX 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("XXX 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("XXX Inconsistent prev\n");
> + do_panic = true;
> + }
> +
> + if (skb == tcp_write_queue_tail(sk)) {
> + if (skb->next != (struct sk_buff *)(&sk->sk_write_queue)) {
> + printk("XXX Improper next at tail\n");
> + do_panic = true;
> + }
> + break;
> + }
> +
> + prev = skb;
> + skb = skb->next;
> + }
> + if (!skb) {
> + printk("XXX Encountered unexpected NULL\n");
> + do_panic = true;
> + }
> + if (do_panic)
> + panic("XXX panicking");
> +}
> +
> /* 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
> @@ -2198,19 +2245,53 @@ 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 unsigned int caught_it;
> +
> 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 *skb, *prev;
> struct sk_buff *hole = NULL;
> + struct sk_buff *old = tp->retransmit_skb_hint;
> u32 last_lost;
> int mib_idx;
> int fwd_rexmitting = 0;
> + bool saw_hint = false;
> +
> + if (!tp->packets_out) {
> + if (net_ratelimit())
> + printk("XXX !tp->packets_out, retransmit_skb_hint=%p, write_queue_head=%p\n",
> + tp->retransmit_skb_hint, tcp_write_queue_head(sk));
> + return;
> + }
>
> if (!tp->lost_out)
> tp->retransmit_high = tp->snd_una;
>
> + for (skb = tcp_write_queue_head(sk),
> + prev = (struct sk_buff *)&sk->sk_write_queue;
> + skb != (struct sk_buff *)&sk->sk_write_queue;
> + prev = skb, skb = skb->next) {
> + if (prev != skb->prev) {
> + printk("XXX sanity check: prev corrupt\n");
> + print_queue(sk, old, hole);
> + }
> + if (skb == tp->retransmit_skb_hint)
> + saw_hint = true;
> + if (skb == tcp_write_queue_tail(sk) &&
> + skb->next != (struct sk_buff *)(&sk->sk_write_queue)) {
> + printk("XXX sanity check: end corrupt\n");
> + print_queue(sk, old, hole);
> + }
> + }
> + if (tp->retransmit_skb_hint && !saw_hint) {
> + printk("XXX sanity check: retransmit_skb_hint=%p is not on list, claring hint\n",
> + tp->retransmit_skb_hint);
> + print_queue(sk, old, hole);
> + tp->retransmit_skb_hint = NULL;
> + }
> +
> if (tp->retransmit_skb_hint) {
> skb = tp->retransmit_skb_hint;
> last_lost = TCP_SKB_CB(skb)->end_seq;
> @@ -2218,7 +2299,17 @@ void tcp_xmit_retransmit_queue(struct sock *sk)
> last_lost = tp->retransmit_high;
> } else {
> skb = tcp_write_queue_head(sk);
> - last_lost = tp->snd_una;
> + if (skb)
> + last_lost = tp->snd_una;
> + }
> +
> +checknull:
> + if (skb == NULL) {
> + print_queue(sk, old, hole);
> + caught_it++;
> + if (net_ratelimit())
> + printk("XXX Errors caught so far %u\n", caught_it);
> + return;
> }
>
> tcp_for_write_queue_from(skb, sk) {
> @@ -2261,7 +2352,7 @@ 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;
> @@ -2272,7 +2363,7 @@ begin_fwd:
> }
>
> if (sacked & (TCPCB_SACKED_ACKED|TCPCB_SACKED_RETRANS))
> - continue;
> + goto cont;
>
> if (tcp_retransmit_skb(sk, skb))
> return;
> @@ -2282,6 +2373,9 @@ 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;
> }
> }
>
>