netdev - RE: [PATCH 1/1] net/hyperv: Fix the code handling tx busy

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Date:	Mon, 19 Mar 2012 17:46:00 +0000
From:	Haiyang Zhang <haiyangz@...rosoft.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	KY Srinivasan <kys@...rosoft.com>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>
Subject: RE: [PATCH 1/1] net/hyperv: Fix the code handling tx busy



> -----Original Message-----
> From: Eric Dumazet [mailto:eric.dumazet@...il.com]
> Sent: Monday, March 19, 2012 1:12 PM
> To: Haiyang Zhang
> Cc: KY Srinivasan; davem@...emloft.net; netdev@...r.kernel.org; linux-
> kernel@...r.kernel.org; devel@...uxdriverproject.org
> Subject: Re: [PATCH 1/1] net/hyperv: Fix the code handling tx busy
> 
> On Mon, 2012-03-19 at 10:02 -0700, Haiyang Zhang wrote:
> > Instead of dropping the packet, we keep the skb buffer, and return
> > NETDEV_TX_BUSY to let upper layer retry send. This will not cause
> > endless loop, because the host is taking data away from ring buffer.
> >
> > Signed-off-by: Haiyang Zhang <haiyangz@...rosoft.com>
> > Reviewed-by: K. Y. Srinivasan <kys@...rosoft.com>
> > ---
> >  drivers/net/hyperv/netvsc_drv.c |    5 +----
> >  1 files changed, 1 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/net/hyperv/netvsc_drv.c
> > b/drivers/net/hyperv/netvsc_drv.c index 2517d20..dd29478 100644
> > --- a/drivers/net/hyperv/netvsc_drv.c
> > +++ b/drivers/net/hyperv/netvsc_drv.c
> > @@ -223,13 +223,10 @@ static int netvsc_start_xmit(struct sk_buff *skb,
> struct net_device *net)
> >  		net->stats.tx_bytes += skb->len;
> >  		net->stats.tx_packets++;
> >  	} else {
> > -		/* we are shutting down or bus overloaded, just drop packet
> */
> > -		net->stats.tx_dropped++;
> >  		kfree(packet);
> > -		dev_kfree_skb_any(skb);
> >  	}
> >
> > -	return NETDEV_TX_OK;
> > +	return ret ? NETDEV_TX_BUSY : NETDEV_TX_OK;
> >  }
> >
> >  /*
> 
> Thats simply not true at all.
> 
> A start_xmit() cannot do that.
> 
> TX_BUSY should never be returned at all, its a deprecated code, for pretty
> good reasons. (assuming queue is not stopped)

We actually stop queue when the ring buffer is busy, see the code in netvsc.c

> Try this on a machine with one CPU, I am pretty sure this can trigger
> complete freezes.

I have tested with one CPU. After NETDEV_TX_BUSY is returned, the Linux
guest OS continues to respond without any problem.

Thanks,
- Haiyang