GFP_ATOMIC vs GFP_KERNEL in netfilter module

[Roberto Jung Drebes]

Hello there,

I am testing a simple netfilter module which delays every 5th IP packet 
received. I am having problems allocating the timer structure. Here is 
the netfilter hook I register:

static unsigned int
comfirm_rx_hook(unsigned int hook,
                struct sk_buff **pskb,
                const struct net_device *in,
                const struct net_device *out,
                int (*okfn) (struct sk_buff *))
{
    static int counter;
    struct timer_list *tl;
    struct cf_delay_parms *dp;

    if ((*pskb)->nfmark != CF_MAGIC) {
        counter++;

        if (!(counter % 5)) {
            dp = kmalloc(sizeof(struct cf_delay_parms), GFP_KERNEL);
            tl = kmalloc(sizeof(struct timer_list), GFP_KERNEL);
            dp->timer = tl;
            (*pskb)->nfmark = CF_MAGIC;
            dp->skb = *pskb;
            tl->data = (unsigned long) dp;
            tl->function = (void *) cf_delay_rx;
            tl->expires = jiffies + 100;
            init_timer(tl);
            add_timer(tl);
            printk("packet delayed\n");
            return NF_STOLEN;
        } else
            return NF_ACCEPT;
    } else
        return NF_ACCEPT;
}

void cf_delay_rx(struct cf_delay_parms *parms)
{
    ip_rcv(parms->skb, parms->skb->dev, NULL);
    kfree(parms->timer);
    kfree(parms);
}

If I use GFP_KERNEL with the kmallocs, I get errors like

Debug: sleeping function called from invalid context at mm/slab.c:2000
in_atomic():1[expected: 0], irqs_disabled():0
 [<0211b765>] __might_sleep+0x82/0x8c
 [<02146dbc>] kmem_cache_alloc+0x1d/0x4c
 [<2204b047>] comfirm_rx_hook+0x47/0xd8 [mymodule]
 [<022a68eb>] nf_iterate+0x40/0x89
 [<022b5d63>] ip_rcv_finish+0x0/0x1d9
 [<022a6bac>] nf_hook_slow+0x47/0xb1
 [<022b5d63>] ip_rcv_finish+0x0/0x1d9
 [<022b5be7>] ip_rcv+0x36d/0x3a1
 [<022b5d63>] ip_rcv_finish+0x0/0x1d9
 [<0229eef3>] netif_receive_skb+0x1b0/0x1dd
 [<0229ef8c>] process_backlog+0x6c/0xd9
 [<0229f056>] net_rx_action+0x5d/0xcd
 [<02122fa1>] __do_softirq+0x35/0x73
 [<02108a7b>] do_softirq+0x46/0x4d
 =======================
 [<02107e67>] do_IRQ+0x2f7/0x303
 [<021fc4c1>] acpi_processor_idle+0xd3/0x1c5
 [<0210408c>] cpu_idle+0x1f/0x34
 [<0239b6ae>] start_kernel+0x221/0x224

If I use GFP_ATOMIC, I don't get the error, but I think timers are not 
being called after the delay. I have a similar code for transmition, 
which works OK with GFP_KERNEL (delays messages) but with GFP_ATOMIC it 
does also not delay.

I test delay with ping, and I am running kernel 2.6.8-1.521 from Fedora 
Core 2.

What am I doing wrong?

TIA,

ps: Please CC me any reply from the linux-kernel mailing list.

--
Roberto Jung Drebes <drebes@xxxxxxxxxxxx>
Porto Alegre, RS - Brasil
http://www.inf.ufrgs.br/~drebes/

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