LLTX fix proposal

LLTX fix proposal

[David S. Miller]

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.

Re: LLTX fix proposal

[jamal]

On Sun, 2005-01-30 at 21:18, David S. Miller wrote:
> 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)
> 

In the e1000 theres a lot more code in hard_start_xmit() that could
be possibly entered before you hit the xmit lock. So thats another
potential issue.
The tg3 you refer to in your email doesnt seem to be as bad and
i am actually wondering why what you just posted is _the fix_ as is.

cheers,
jamal

Re: LLTX fix proposal

[jamal]

On Sun, 2005-01-30 at 22:33, jamal wrote:
> On Sun, 2005-01-30 at 21:18, David S. Miller wrote:
> > 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)
> > 
> 
> In the e1000 theres a lot more code in hard_start_xmit() that could
> be possibly entered before you hit the xmit lock. So thats another
> potential issue.
> The tg3 you refer to in your email doesnt seem to be as bad and
> i am actually wondering why what you just posted is _the fix_ as is.

meant is not _the fix_
too many typos - off to bed.

cheers,
jamal

Re: LLTX fix proposal

[David S. Miller]

On 30 Jan 2005 22:38:00 -0500
jamal <hadi@cyberus.ca> wrote:

> On Sun, 2005-01-30 at 22:33, jamal wrote:
> > The tg3 you refer to in your email doesnt seem to be as bad and
> > i am actually wondering why what you just posted is _the fix_ as is.
> 
> meant is not _the fix_

It could be "_the fix_", I just don't know yet if we'll be
deeply satisfied with it in the end.  That's all.