Subject: Re: [PATCH] Prevent netpoll hanging when link is down

From: "David S. Miller" <[email-masked]>
Date: Thu, 7 Oct 2004 11:28:46 -0700

On Thu, 7 Oct 2004 16:05:32 +0200
Colin Leroy <[email-masked]> wrote:

> First, my newbie question: is it possible to deadlock a spinlock on a
> Uniprocessor kernel ? For example, there's something I find suspect in
> netpoll/sungem interaction:
> 

Oh yes, it appears that netpoll doesn't support NETIF_F_LLTX locking,
crap :(

When a device has NETIF_F_LLTX set, it means that the driver's
dev->hard_start_xmit() routine is what takes the xmit_lock, not
the caller one level up.

Andi Kleen didn't fix up netpoll when he did his LLTX changes, oops.

So, netpoll needs to have the NETIF_F_LLTX stuff added to it.
Basically:

1) If NETIF_F_LLTX is clear, same as before
2) If NETIF_F_LLTX is set:
	a) Do not take xmit_lock
	b) Check ->hard_start_xmit() return value,
	   if it is NETDEV_TX_LOCKED, then
	   spin_trylock(&dev->xmit_lock) failed
           in ->hard_start_xmit()

The best example is in net/sched/sch_generic.c:qdisc_restart()

		unsigned nolock = (dev->features & NETIF_F_LLTX);
		/*
		 * When the driver has LLTX set it does its own locking
		 * in start_xmit. No need to add additional overhead by
		 * locking again. These checks are worth it because
		 * even uncongested locks can be quite expensive.
		 * The driver can do trylock like here too, in case
		 * of lock congestion it should return -1 and the packet
		 * will be requeued.
		 */
		if (!nolock) {
			if (!spin_trylock(&dev->xmit_lock)) {
			collision:
				/* So, someone grabbed the driver. */
				
				/* It may be transient configuration error,
				   when hard_start_xmit() recurses. We detect
				   it by checking xmit owner and drop the
				   packet when deadloop is detected.
				*/
				if (dev->xmit_lock_owner == smp_processor_id()) {
					kfree_skb(skb);
					if (net_ratelimit())
						printk(KERN_DEBUG "Dead loop on netdevice %s, fix it urgently!\n", dev->name);
					return -1;
				}
				__get_cpu_var(netdev_rx_stat).cpu_collision++;
				goto requeue;
			}
			/* Remember that the driver is grabbed by us. */
			dev->xmit_lock_owner = smp_processor_id();
		}
		
		{
			/* And release queue */
			spin_unlock(&dev->queue_lock);

			if (!netif_queue_stopped(dev)) {
				int ret;
				if (netdev_nit)
					dev_queue_xmit_nit(skb, dev);

				ret = dev->hard_start_xmit(skb, dev);
				if (ret == NETDEV_TX_OK) { 
					if (!nolock) {
						dev->xmit_lock_owner = -1;
						spin_unlock(&dev->xmit_lock);
					}
					spin_lock(&dev->queue_lock);
					return -1;
				}
				if (ret == NETDEV_TX_LOCKED && nolock) {
					spin_lock(&dev->queue_lock);
					goto collision; 
				}
			}

			/* NETDEV_TX_BUSY - we need to requeue */
			/* Release the driver */
			if (!nolock) { 
				dev->xmit_lock_owner = -1;
				spin_unlock(&dev->xmit_lock);
			} 
			spin_lock(&dev->queue_lock);
			q = dev->qdisc;
		}