netdev - Re: [net-2.6 PATCH 2/2] net: decreasing real_num_tx

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Date:	Fri, 02 Jul 2010 21:59:57 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	jeffrey.t.kirsher@...el.com
Cc:	netdev@...r.kernel.org, gospo@...hat.com, bphilips@...ell.com,
	john.r.fastabend@...el.com
Subject: Re: [net-2.6 PATCH 2/2] net: decreasing real_num_tx_queues needs
 to flush qdisc

From: Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Date: Thu, 01 Jul 2010 16:21:57 -0700

> From: John Fastabend <john.r.fastabend@...el.com>
> 
> Reducing real_num_queues needs to flush the qdisc otherwise
> skbs with queue_mappings greater then real_num_tx_queues can
> be sent to the underlying driver.
> 
> The flow for this is,
> 
> dev_queue_xmit()
> 	dev_pick_tx()
> 		skb_tx_hash()  => hash using real_num_tx_queues
> 		skb_set_queue_mapping()
> 	...
> 	qdisc_enqueue_root() => enqueue skb on txq from hash
> ...
> dev->real_num_tx_queues -= n
> ...
> sch_direct_xmit()
> 	dev_hard_start_xmit()
> 		ndo_start_xmit(skb,dev) => skb queue set with old hash
> 
> skbs are enqueued on the qdisc with skb->queue_mapping set
> 0 < queue_mappings < real_num_tx_queues.  When the driver
> decreases real_num_tx_queues skb's may be dequeued from the
> qdisc with a queue_mapping greater then real_num_tx_queues.
> 
> This fixes a case in ixgbe where this was occurring with DCB
> and FCoE. Because the driver is using queue_mapping to map
> skbs to tx descriptor rings we can potentially map skbs to
> rings that no longer exist.
> 
> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
> Tested-by: Ross Brattain <ross.b.brattain@...el.com>
> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@...el.com>

Applied.
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