Subject: Re: [sungem] proposal for a new locking strategy

From: Stephen Hemminger <[email-masked]>
Date: Sun, 5 Nov 2006 09:41:36 -0800

On Sun, 5 Nov 2006 18:28:33 +0100
"Eric Lemoine" <[email-masked]> wrote:

> > You could also just use net_tx_lock() now.
> 
> You mean netif_tx_lock()?
> 
> Thanks for letting me know about that function. Yes, I may need it.
> tg3 and bnx2 use it to wake up the transmit queue:
> 
>          if (unlikely(netif_queue_stopped(tp->dev) &&
>                       (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH))) {
>                  netif_tx_lock(tp->dev);
>                  if (netif_queue_stopped(tp->dev) &&
>                      (tg3_tx_avail(tp) > TG3_TX_WAKEUP_THRESH))
>                          netif_wake_queue(tp->dev);
>                  netif_tx_unlock(tp->dev);
>          }
> 
> 2.6.17 didn't use it. Was it a bug?
> 
> Thanks,

No, it was introduced in 2.6.18. The functions are just a wrapper
around the network device transmit lock that is normally held.

If the device does not need to acquire the lock during IRQ, it
is a good alternative and avoids a second lock.

For transmit locking there are three common alternatives:

Method A: dev->queue_xmit_lock and per-device tx_lock
	send: dev->xmit_lock held by caller
	    dev->hard_start_xmit acquires netdev_priv(dev)->tx_lock

	irq:  netdev_priv(dev)->tx_lock acquired

Method B: dev->queue_xmit_lock only
	send: dev->xmit_lock held by caller
	irq:  schedules softirq (NAPI)
	napi_poll: calls netif_tx_lock() which acquires dev->xmit_lock

Method C: LLTX
	set dev->features LLTX
	send: no locks held by caller
		dev->hard_start_xmit acquires netdev_priv(dev)->tx_lock
	irq: netdev_priv(dev)->tx_lock acquired

Method A is the only one that works with 2.4 and early (2.6.8?) kernels.