netdev - Re: [PATCH net-next-2.6] net: Xmit Packet Steering (XPS)

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Date:	Fri, 20 Nov 2009 23:08:51 +0100
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	Linux Netdev List <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next-2.6] net: Xmit Packet Steering (XPS)

On Fri, Nov 20, 2009 at 10:43:55PM +0100, Eric Dumazet wrote:
> Jarek Poplawski a écrit :
> > On Fri, Nov 20, 2009 at 03:45:42PM +0100, Eric Dumazet wrote:
> >> Jarek Poplawski a écrit :
> >>> On 20-11-2009 00:46, Eric Dumazet wrote:
> >>> scheduled) IPIs?
> >> Repeat after me :
> >>
> >> lockdep is my friend, lockdep is my friend, lockdep is my friend... :)
> > 
> > Hmm... Actually, why did I have to do lockdep's job...
> 
> In fact I could not find why irq masking is necessary, and lockdep is
> fine with my code and my testings. Care to explain what problem you spotted ?
> 

CPU1						 CPU2
net_rx_action()					 net_rx_action()
 xps_flush()				  	  xps_flush()
  q = &per_cpu(xps_pcpu_queue, cpu2)		   q = &per_cpu(xps_pcpu_queue, cpu1)
  spin_lock(&q->list.lock of cpu2)		   spin_lock(&q->list.lock of cpu1)

<IPI>						 <IPI>
remote_free_skb_list()				 remote_free_skb_list()
waiting on spin_lock(&q->list.lock of cpu1)	 waiting on spin_lock(&q->list.lock of cpu2)


IPIs triggerered e.g. by CPU3 (or/and CPU4...) doing net_rx_action as well.

Jarek P.
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