netdev - Re: tun: Use netif_receive_skb instead of netif

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Date:	Wed, 19 May 2010 17:00:47 -0400
From:	Brian Bloniarz <bmb@...enacr.com>
To:	tgraf@...hat.com
CC:	Neil Horman <nhorman@...driver.com>,
	Neil Horman <nhorman@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: tun: Use netif_receive_skb instead of netif_rx

On 05/19/2010 04:49 PM, Thomas Graf wrote:
> On Wed, 2010-05-19 at 14:00 -0400, Neil Horman wrote: 
>> I'm currently testing this, unfortunately, and its not breaking anything, but it
>> doesn't allow cgroups to classify frames comming from tun interfaces.  I'm still
>> investigating, but I think the issue is that, because we call local_bh_disable
>> with this patch, we wind up raising the count at SOFTIRQ_OFFSET in preempt_count
>> for the task.  Since the cgroup classifier has this check:
>>
>> if (softirq_count() != SOFTIRQ_OFFSET))
>> 	return -1;
>>
>> We still fail to classify the frame.  the cgroup classifier is assuming that any
>> frame arriving with a softirq count of 1 means we came directly from the
>> dev_queue_xmit routine and is safe to check current().  Any less than that, and
>> something is wrong (as we at least need the local_bh_disable in dev_queue_xmit),
>> and any more implies that we have nested calls to local_bh_disable, meaning
>> we're really handling a softirq context.
> 
> It is a hack but the only method to check for softirq context I found. I
> would favor using a flag if there was one.

Eric probably has some thoughts on this -- his scheduler-batching patch RFC
from last year needed the same bit of info:
http://patchwork.ozlabs.org/patch/24536/
(see the changes to trace_softirq_context).
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