[PATCH] 8139too deadlock fix

> stack backtrace:

>  [<f9099d31>] rtl8139_start_xmit+0xd9/0xff [8139too]
>  [<c11ad5ea>] netpoll_send_skb+0x98/0xea

This seems to be a real deadlock...

So netpoll_send_skb takes the _xmit_lock, which is all nitty gritty
but then rtl8139_start_xmit comes around while that lock is taken, and
does

      spin_unlock_irq(&tp->lock);

which.. enables interrupts and softirqs; this is quite bad because the
xmit lock is taken in softirq context for the watchdog like this:
  [<c1200376>] _spin_lock+0x23/0x32
  [<c11af282>] dev_watchdog+0x14/0xb1
  [<c101dab2>] run_timer_softirq+0xf2/0x14a
  [<c101a691>] __do_softirq+0x55/0xb0
  [<c1004a8d>] do_softirq+0x58/0xbd

Which would deadlock now that the spin_unlock_irq() has enabled
irqs/softirqs while the _xmit_lock is still held.

The patch below turns this into a irqsave/irqrestore pair so that
interrupts don't get enabled unconditionally.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/8139too.c

@@ -1709,6 +1709,7 @@ static int rtl8139_start_xmit (struct sk_buff *skb, struct net_device *dev)
 	void __iomem *ioaddr = tp->mmio_addr;
 	unsigned int entry;
 	unsigned int len = skb->len;
+	unsigned long flags;
 
 	/* Calculate the next Tx descriptor entry. */
 	entry = tp->cur_tx % NUM_TX_DESC;
@@ -1725,7 +1726,7 @@ static int rtl8139_start_xmit (struct sk_buff *skb, struct net_device *dev)
 		return 0;
 	}
 
-	spin_lock_irq(&tp->lock);
+	spin_lock_irqsave(&tp->lock, flags);
 	RTL_W32_F (TxStatus0 + (entry * sizeof (u32)),
 		   tp->tx_flag | max(len, (unsigned int)ETH_ZLEN));
 
@@ -1736,7 +1737,7 @@ static int rtl8139_start_xmit (struct sk_buff *skb, struct net_device *dev)
 
 	if ((tp->cur_tx - NUM_TX_DESC) == tp->dirty_tx)
 		netif_stop_queue (dev);
-	spin_unlock_irq(&tp->lock);
+	spin_unlock_irqrestore(&tp->lock, flags);
 
 	if (netif_msg_tx_queued(tp))
 		printk (KERN_DEBUG "%s: Queued Tx packet size %u to slot %d.\n",