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

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Date:	Tue, 13 Apr 2010 16:57:56 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Stephen Hemminger <shemminger@...ux-foundation.org>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Jay Vosburgh <fubar@...ibm.com>,
	Neil Horman <nhorman@...driver.com>, netdev@...r.kernel.org,
	Matt Mackall <mpm@...enic.com>,
	bridge@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Jeff Moyer <jmoyer@...hat.com>,
	Andy Gospodarek <gospo@...hat.com>,
	bonding-devel@...ts.sourceforge.net
Subject: Re: [Bonding-devel] [v3 Patch 2/3] bridge: make bridge support netpoll

Stephen Hemminger wrote:
> On Mon, 12 Apr 2010 12:38:57 +0200
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
>> Le lundi 12 avril 2010 à 18:37 +0800, Cong Wang a écrit :
>>> Stephen Hemminger wrote:
>>>> 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...
>> Yes, its RTNL that protects priv_flags changes, hopefully...
>>
> 
> The patch was not protecting priv_flags with RTNL.
> For example..
> 
> 
> @@ -308,7 +312,9 @@ static void netpoll_send_skb(struct netp
>  		     tries > 0; --tries) {
>  			if (__netif_tx_trylock(txq)) {
>  				if (!netif_tx_queue_stopped(txq)) {
> +					dev->priv_flags |= IFF_IN_NETPOLL;
>  					status = ops->ndo_start_xmit(skb, dev);
> +					dev->priv_flags &= ~IFF_IN_NETPOLL;
>  					if (status == NETDEV_TX_OK)
>  						txq_trans_update(txq);

Hmm, but I checked the bonding case (IFF_BONDING), it doesn't
hold rtnl_lock. Strange.

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