LLTX fix proposal

["David S. Miller" <davem@xxxxxxxxxxxxx>]

Ok, how about we do this for 2.6.11 before it goes out and
do something more sophisticated (if necessary) later.

The BUG() we're trying to catch in the ->hard_start_xmit()
routines is the illegal state:

        driver_tx_queue_empty() && !netif_queue_stopped(dev)

Therefore we could handle the race, and avoid the printk() in
the race case but not in the BUG() case above.  The test in
tg3.c is currently:

        /* This is a hard error, log it. */
        if (unlikely(TX_BUFFS_AVAIL(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
                netif_stop_queue(dev);
                spin_unlock_irqrestore(&tp->tx_lock, flags);
                printk(KERN_ERR PFX "%s: BUG! Tx Ring full when queue awake!\n",
                       dev->name);
                return NETDEV_TX_BUSY;
        }

and I'm proposing we change it to something like:

        if (unlikely(TX_BUFFS_AVAIL(tp) <= (skb_shinfo(skb)->nr_frags + 1))) {
                /* We can race with queue processing on another
                 * cpu due to LLTX.  If the queue is not stopped,
                 * that is a hard error, log it.
                 */
                if (!netif_queue_stopped(dev)) {
                        netif_stop_queue(dev);
                        printk(KERN_ERR PFX "%s: BUG! Tx Ring full when "
                               "queue awake!\n",
                               dev->name);
                }
                spin_unlock_irqrestore(&tp->tx_lock, flags);
                return NETDEV_TX_BUSY;
        }

Any objections?

Again, I'm not saying %100 this is what we should do long-term.
It's meant to be correct and eliminate the bogus log messages
when the LLTX race is hit.