netdev - Re: [PATCH] bonding: reset queue mapping prior to transmission to physical device (v4)

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Date:	Fri, 3 Jun 2011 15:57:00 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Jay Vosburgh <fubar@...ibm.com>,
	Andy Gospodarek <andy@...yhouse.net>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH] bonding: reset queue mapping prior to transmission to
 physical device (v4)

On Fri, Jun 03, 2011 at 09:48:51PM +0200, Eric Dumazet wrote:
> Le vendredi 03 juin 2011 à 15:24 -0400, Neil Horman a écrit :
> > The bonding driver is multiqueue enabled, in which each queue represents a slave
> > to enable optional steering of output frames to given slaves against the default
> > output policy.  However, it needs to reset the skb->queue_mapping prior to
> > queuing to the physical device or the physical slave (if it is multiqueue) could
> > wind up transmitting on an unintended tx queue
> > 
> > Change Notes:
> > v2) Based on first pass review, updated the patch to restore the origional queue
> > mapping that was found in bond_select_queue, rather than simply resetting to
> > zero.  This preserves the value of queue_mapping when it was set on receive in
> > the forwarding case which is desireable.
> > 
> > v3) Fixed spelling an casting error in skb->cb
> > 
> > v4) fixed to store raw queue_mapping to avoid double decrement
> > 
> > Signed-off-by: Neil Horman <nhorman@...driver.com>
> > CC: Jay Vosburgh <fubar@...ibm.com>
> > CC: Andy Gospodarek <andy@...yhouse.net>
> > CC: "David S. Miller" <davem@...emloft.net>
> > ---
> >  drivers/net/bonding/bond_main.c |    8 ++++++++
> >  1 files changed, 8 insertions(+), 0 deletions(-)
> > 
> > diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> > index 17b4dd9..76adf27 100644
> > --- a/drivers/net/bonding/bond_main.c
> > +++ b/drivers/net/bonding/bond_main.c
> > @@ -400,6 +400,9 @@ int bond_dev_queue_xmit(struct bonding *bond, struct sk_buff *skb,
> >  {
> >  	skb->dev = slave_dev;
> >  	skb->priority = 1;
> > +
> > +	skb->queue_mapping = ((u16 *)skb->cb)[0];
> 
> Please dont do that. Use a helper.
> 
Why?  It seems to be reasonably common practice for drivers to access
queue_mapping directly.

Examples can be found in:
ixgbe_xmit_frame
mlx4_en_xmit
qlge_send
igb_xmit_frame_adv
gfar_start_xmit

among others.

Not saying its correct to do so necessecarily, but it seems a helper doesn't buy
us much here, particularly a per-driver helper.  If a helper really should be
used, why not just consistently use skb_get_queue_mapping?
Neil
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