Re: tcp crash in net-2.6 tree

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 30 Mar 2007 17:33:28 +0300 (EEST) "Ilpo Järvinen" <ilpo.jarvinen@helsinki.fi> wrote:

> On Thu, 29 Mar 2007, Andrew Morton wrote:
> 
> > Full -mm lineup.  The x86_64 box was acting as a distcc server at the time.
> > 
> > Nothing hit the logs, I'm afraid. But almost all the info is in 
> > http://userweb.kernel.org/~akpm/s5000494.jpg
> >
> > <pokes around in gdb a bit>
> > 
> > It died in tcp_update_scoreboard_fack() here:
> > 
> >                 if ((!IsFack(tp) || !tcp_skb_timedout(sk, skb)) &&
> >                     after(TCP_SKB_CB(skb)->seq, tp->high_seq)) {
> >                         /* RFC: should we have find_below? */
> >                         skb = tcp_write_queue_find(sk, tp->high_seq);
> >                         not_marked_skb = skb;
> >                         skb = tcp_write_queue_prev(sk, skb);
> >                         /* Timedout top is again uncertain? */
> >                         if (tcp_skb_timedout(sk, skb))
> >                                 timedout_continue = 1;
> >                 }
> > 
> > 
> > (gdb) l *0xffffffff8048ded8
> > 0xffffffff8048ded8 is in tcp_update_scoreboard_fack (include/net/tcp.h:1197).
> > 1192            return skb->next;
> > 1193    }
> > 1194    
> > 1195    static inline struct sk_buff *tcp_write_queue_prev(struct sock *sk, struct sk_buff *skb)
> > 1196    {
> > 1197            return skb->prev;
> > 1198    }
> > 1199    
> > 1200    #define tcp_for_write_queue(skb, sk)                                   \
> > 
> > (gdb) x/10i 0xffffffff8048ded8
> > 0xffffffff8048ded8 <tcp_update_scoreboard_fack+528>:    mov    0x8(%rdx),%r12
> > 
> > So if that "CR2: 0000000000000008" is the access address then it appears that
> > `skb' is NULL.  tcp_write_queue_find() didn't find nuthin.
> 
> If there is nothing at high_seq (application hasn't given any data to/past 
> that point), the search fails to find any skb and returns NULL... But I 
> have no idea how this can happen? As TCP does after(skb->seq, 
> tp->high_seq) (even in the quoted code block) guaranteeing that something 
> is there after the high_seq for TCP to step temporarily on... So at least 
> one skb should have it's end_seq after tp->high_seq (actually there 
> should be at least two valid skbs after tp->high_seq since the used 
> sequence number space does not have holes), which should be enough to get 
> an existing skb from write_queue_find?!
> 
> I also checked all call paths to tcp_update_scoreboard_fack to make sure 
> that snd_una hasn't gone past high_seq and found nothing suspicious (and 
> that wouldn't return NULL anyway I think)...

Well, could I suggest that you prepare a patch which adds lots of debugging
checks in that area?  I can apply it and will rerun the same workload.