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    SubjectRe: Revert "gro: Fix legacy path napi_complete crash",
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    From: Herbert Xu <herbert@gondor.apana.org.au>
    Date: Tue, 24 Mar 2009 23:09:28 +0800

    > On Tue, Mar 24, 2009 at 03:39:42PM +0100, Ingo Molnar wrote:
    > >
    > > Subject: [PATCH] net: Fix netpoll lockup in legacy receive path
    >
    > Actually, this patch is still racy. If some interrupt comes in
    > and we suddenly get the maximum amount of backlog we can still
    > hang when we call __napi_complete incorrectly. It's unlikely
    > but we certainly shouldn't allow that. Here's a better version.
    >
    > net: Fix netpoll lockup in legacy receive path

    Hmmm...

    > @@ -2588,9 +2588,10 @@ static int process_backlog(struct napi_struct *napi, int quota)
    > local_irq_disable();
    > skb = __skb_dequeue(&queue->input_pkt_queue);
    > if (!skb) {
    > + list_del(&napi->poll_list);
    > + clear_bit(NAPI_STATE_SCHED, &napi->state);
    > local_irq_enable();
    > - napi_complete(napi);
    > - goto out;
    > + break;
    > }
    > local_irq_enable();

    I think the problem is that we need to do the GRO flush before the
    list delete and clearing the NAPI_STATE_SCHED bit.

    You can't disown the NAPI context until you've squared away the GRO
    state, I think.

    Ingo's case stresses TCP a lot so I think he's hitting these GRO
    cases a lot as well as hitting the backlog maximum.

    So this mis-ordering of completion operations could explain why
    he still sees problems.


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