A question on "Scheduling in interrupt".

A question on "Scheduling in interrupt".

[卫清]

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Dear Networking Teak,

I am a system engineer in a Chinese software development company, and currently I am implementing IPSec on Linux version 2.2.16. Generally speaking, what I am doing is adding internet key exchange and IPsec processing functions into the Linux ip_queue_xmit, ip_build_xmit and ip_local_deliver procedures. After I have built up a new sk_buff variable, I send it off using my own function send_skbuff(), this is where the problem occurs. Following is the complete copy of the culprit:

void send_skbuff(struct sk_buff *skb)
{
  struct rtable *rt;
  struct iphdr *iph;

  iph = skb->nh.iph;
  if(ip_route_output(&rt, iph->daddr, iph->saddr,
 IPTOS_LOWDELAY, 0))
    goto drop;
  skb->dst = dst_clone(&rt->u.dst);
  skb->priority = TC_PRIO_FILLER;
  if (skb_cloned(skb))
    skb = skb_copy(skb, GFP_ATOMIC);
  else
    skb = skb_clone(skb, GFP_ATOMIC);
  if (skb->len > rt->u.dst.pmtu)
    goto fragment;
  skb->dst->output(skb);
  return;
fragment:
  ip_fragment(skb, skb->dst->output);
  return;
drop:
  kfree_skb(skb); 
}

All functions within the above procedure are kernel supplied and irrelevant to the problem, so I will skip its explanation.
I use the kgdb to debug my codes. Everything worked fine until I had crossed the skb->dst->output(skb) statement and hit upon return. Then, when I typed “next” on host machine, the system crashed with “Scheduling in interrupt!” appearing on target machine and the following message on my host machine:

Program received signal SIGSEGV, Segmentation fault.
Schedule () at sched.c: 875
875 *(int *)0 = 0

Leery of some low level codes within the “skb->dst->output(skb)” provoked the crash, I step into that statement which points to function ip_output. Alas, the system crashed with the same reason upon the first statement of procedure ip_output, and that statement does simple book keeping!
The problem is patent: scheduling while within the interrupt processing, the myth is how could it ever be scheduling within interrupt since I am not doing anything within the interrupt context!? It is a well known fact that within Linux there are three places to evoke scheduling, either it is educed explicitly by the procedure itself, which won’t be within interrupt context as long as the caller is not within it; or it can be elicited when the current process’s time quantum is exhausted or when a newly awakened process has a higher priority than the current one, during these two situations, the system will mark the need_resched flag, and the actual scheduling is performed when the system returns from a interrupt, again within the context of the process that is to be scheduled away. Therefore normally there is no chance to be scheduling in interrupt. Where is the hole? The problem must be caused by my codes, shouldn’t it?
By the way, I implemented the send_skbuff months ago, and together with other codes they had been working fine. This scheduling in interrupt problem only recently popped up when I was debugging error processing for IPsec and the place it occurred is irrelevant to the error processing.
Any comments will be greatly appreciated.

Regards,

Lee Tong


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