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

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Date:	Mon, 12 Apr 2010 08:38:42 -0700
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Cong Wang <amwang@...hat.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

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);
--
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