On Fri, 30 Mar 2007 17:33:28 +0300 (EEST) "Ilpo Järvinen" [off-list ref] wrote:

On Thu, 29 Mar 2007, Andrew Morton wrote:

quoted

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.