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Date:	Thu, 15 Apr 2010 15:30:44 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	eric.dumazet@...il.com, therbert@...gle.com, eparis@...hat.com,
	netdev@...r.kernel.org
Subject: Re: BUG: using smp_processor_id() in preemptible [00000000] code: 
	avahi-daemon: caller is netif_rx

On Thu, Apr 15, 2010 at 3:14 PM, David Miller <davem@...emloft.net> wrote:
> From: Eric Dumazet <eric.dumazet@...il.com>
> Date: Tue, 13 Apr 2010 09:14:17 +0200
>
>> [PATCH net-next-2.6] net: netif_rx() must disable preemption
>>
>> Eric Paris reported netif_rx() is calling smp_processor_id() from
>> preemptible context, in particular when caller is
>> ip_dev_loopback_xmit().
>>
>> RPS commit added this smp_processor_id() call, this patch makes sure
>> preemption is disabled. rps_get_cpus() wants rcu_read_lock() anyway, we
>> can dot it a bit earlier.
>>
>> Reported-by: Eric Paris <eparis@...hat.com>
>> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
>
> I've applied this with some coding style fixups.
>
> Thanks!
>
> --------------------
> net: netif_rx() must disable preemption
>
> Eric Paris reported netif_rx() is calling smp_processor_id() from
> preemptible context, in particular when caller is
> ip_dev_loopback_xmit().
>
> RPS commit added this smp_processor_id() call, this patch makes sure
> preemption is disabled. rps_get_cpus() wants rcu_read_lock() anyway, we
> can dot it a bit earlier.
>
> Reported-by: Eric Paris <eparis@...hat.com>
> Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> ---
>  net/core/dev.c |   25 +++++++++++++++----------
>  1 files changed, 15 insertions(+), 10 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 876b111..e8041eb 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -2206,6 +2206,7 @@ DEFINE_PER_CPU(struct netif_rx_stats, netdev_rx_stat) = { 0, };
>  /*
>  * get_rps_cpu is called from netif_receive_skb and returns the target
>  * CPU from the RPS map of the receiving queue for a given skb.
> + * rcu_read_lock must be held on entry.
>  */
>  static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb)
>  {
> @@ -2217,8 +2218,6 @@ static int get_rps_cpu(struct net_device *dev, struct sk_buff *skb)
>        u8 ip_proto;
>        u32 addr1, addr2, ports, ihl;
>
> -       rcu_read_lock();
> -
>        if (skb_rx_queue_recorded(skb)) {
>                u16 index = skb_get_rx_queue(skb);
>                if (unlikely(index >= dev->num_rx_queues)) {
> @@ -2296,7 +2295,6 @@ got_hash:
>        }
>
>  done:
> -       rcu_read_unlock();
>        return cpu;
>  }
>
> @@ -2392,7 +2390,7 @@ enqueue:
>
>  int netif_rx(struct sk_buff *skb)
>  {
> -       int cpu;
> +       int ret;
>
>        /* if netpoll wants it, pretend we never saw it */
>        if (netpoll_rx(skb))
> @@ -2402,14 +2400,21 @@ int netif_rx(struct sk_buff *skb)
>                net_timestamp(skb);
>
>  #ifdef CONFIG_RPS
> -       cpu = get_rps_cpu(skb->dev, skb);
> -       if (cpu < 0)
> -               cpu = smp_processor_id();
> +       {
> +               int cpu;
> +
> +               rcu_read_lock();
> +               cpu = get_rps_cpu(skb->dev, skb);
> +               if (cpu < 0)
> +                       cpu = smp_processor_id();
> +               ret = enqueue_to_backlog(skb, cpu);
> +               rcu_read_unlock();
> +       }
>  #else
> -       cpu = smp_processor_id();
> +       ret = enqueue_to_backlog(skb, get_cpu());
> +       put_cpu();
>  #endif
> -
> -       return enqueue_to_backlog(skb, cpu);
> +       return ret;
>  }
>  EXPORT_SYMBOL(netif_rx);
>

Should netif_rx() be used only when preemption is disabled? If not,
netif_rx_ni() should be used instead.?


-- 
Regards,
Changli Gao(xiaosuo@...il.com)
--
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