netdev - Re: [PATCH] net/hyperv: fix erroneous NETDEV_TX

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Date:	Wed, 14 Mar 2012 12:17:12 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Greear <greearb@...delatech.com>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] net/hyperv: fix erroneous NETDEV_TX_BUSY use

On Wed, 2012-03-14 at 11:56 -0700, Ben Greear wrote:
> On 03/14/2012 11:53 AM, Eric Dumazet wrote:
> > A driver start_xmit() method cannot free skb and return NETDEV_TX_BUSY,
> > since caller is going to reuse freed skb.
> >
> > This is mostly a revert of commit bf769375c (staging: hv: fix the return
> > status of netvsc_start_xmit())
> >
> > In fact netif_tx_stop_queue() / netif_stop_queue() is needed before
> > returning NETDEV_TX_BUSY or you can trigger a ksoftirqd fatal loop.
> >
> > In case of memory allocation error, only safe way is to drop the packet
> > and return NETDEV_TX_OK
> 
> Can you not just leave the skb alone, not bump any tx-dropped
> stats, and return NETDEV_TX_BUSY?
> 

Nope. Think about OOM.

If we do that, we requeue packet on qdisc, and schedule TX softirq.

-> loop

Really, NETDEV_TX_BUSY is not for this kind of situation.



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