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    SubjectRe: 2.4.4: Kernel crash, possibly tcp related
    On Tue, May 01, 2001 at 08:44:52PM +0400, wrote:
    > Hello!
    > > this is the strict fix:
    > Andrea, you caught the problem!
    > The fix is not right though (it is equivalent to straight
    > tp->send_head=NULL, as you noticed. It also corrupts queue in
    > an opposite manner.) Right fix is appended.
    > Explanation: in do_fault we must undo effect of enqueueing new segment
    > in the case the segment remained empty. tp->send_head points to
    > the first unsent skb in queue and it is NULL when and only when
    > all the skbs are already sent. (Invariant is: tp->send_head==NULL ||
    > tp->send_head->seq == tp->snd_nxt)
    > I crapped this case except for the case when queue is completely empty,
    > so that the last sent skb was accounted in packets_out twice...

    I understsand the explanation but I don't think my patch is wrong, I
    think it's simpler and faster instead.

    My argument is very simple, if send_head points to skb and skb->len is
    zero and we are running in such slow path, it is obvious the send_head
    _was_ NULL when we entered the critical section, so it's perfectly fine
    to set send_head back to null and to unlink the skb as the only actions
    to undo the skb_entail. That's all. I don't see how my patch can fail.
    If I'm missing something I'd love a further explanation indeed. Thanks!

    > Damn, what a silly mistake was it... shame.
    > Alexey
    > --- ../vger3-010426/linux/net/ipv4/tcp.c Wed Apr 25 21:02:18 2001
    > +++ linux/net/ipv4/tcp.c Tue May 1 20:38:44 2001
    > @@ -1185,7 +1187,7 @@
    > if (skb->len==0) {
    > if (tp->send_head == skb) {
    > tp->send_head = skb->prev;
    > - if (tp->send_head == (struct sk_buff*)&sk->write_queue)
    > + if (TCP_SKB_CB(skb)->seq == tp->snd_nxt)
    > tp->send_head = NULL;
    > }
    > __skb_unlink(skb, skb->list);

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