netdev - Re: [v3 Patch 2/3] bridge: make bridge support netpoll

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Message-ID: <4BC2F7E2.7020309@redhat.com>
Date:	Mon, 12 Apr 2010 18:37:22 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	bridge@...ts.linux-foundation.org,
	Andy Gospodarek <gospo@...hat.com>,
	Neil Horman <nhorman@...driver.com>,
	Jeff Moyer <jmoyer@...hat.com>, Matt Mackall <mpm@...enic.com>,
	bonding-devel@...ts.sourceforge.net,
	Jay Vosburgh <fubar@...ibm.com>,
	David Miller <davem@...emloft.net>
Subject: Re: [v3 Patch 2/3] bridge: make bridge support netpoll

Stephen Hemminger wrote:
>> Index: linux-2.6/net/bridge/br_forward.c
>> ===================================================================
>> --- linux-2.6.orig/net/bridge/br_forward.c
>> +++ linux-2.6/net/bridge/br_forward.c
>> @@ -15,6 +15,7 @@
>>  #include <linux/slab.h>
>>  #include <linux/kernel.h>
>>  #include <linux/netdevice.h>
>> +#include <linux/netpoll.h>
>>  #include <linux/skbuff.h>
>>  #include <linux/if_vlan.h>
>>  #include <linux/netfilter_bridge.h>
>> @@ -50,7 +51,13 @@ int br_dev_queue_push_xmit(struct sk_buf
>>  		else {
>>  			skb_push(skb, ETH_HLEN);
>>  
>> -			dev_queue_xmit(skb);
>> +#ifdef CONFIG_NET_POLL_CONTROLLER
>> +			if (skb->dev->priv_flags & IFF_IN_NETPOLL) {
>> +				netpoll_send_skb(skb->dev->npinfo->netpoll, skb);
>> +				skb->dev->priv_flags &= ~IFF_IN_NETPOLL;
>> +			} else
>> +#endif
> 
> There is no protection on dev->priv_flags for SMP access.
> It would better bit value in dev->state if you are using it as control flag.
> 
> Then you could use 
> 			if (unlikely(test_and_clear_bit(__IN_NETPOLL, &skb->dev->state)))
> 				netpoll_send_skb(...)
> 
> 

Hmm, I think we can't use ->state here, it is not for this kind of purpose,
according to its comments.

Also, I find other usages of IFF_XXX flags of ->priv_flags are also using
&, | to set or clear the flags. So there must be some other things preventing
the race...


Thanks.
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