linux-kernel - Re: [PATCH 1/3] net: lpc_eth: Replace WARN() trace with simple pr

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Date:	Mon, 11 Jun 2012 10:53:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Roland Stigge <stigge@...com.de>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, kevin.wells@....com,
	srinivas.bakki@....com, aletes.xgr@...il.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 1/3] net: lpc_eth: Replace WARN() trace with simple
 pr_warn()

On Mon, 2012-06-11 at 10:36 +0200, Roland Stigge wrote:

> I encountered cases where this happened for me on a custom board under
> heavy load.
> 
> I discussed this with Kevin Wells, the original driver author. We
> identified the case of xmit()'s TX request (from .ndo_start_xmit) with
> full TX driver buffers as valid when ethernet is busy.
> 
> But maybe this is wrong. Can you please give me a hint how the net
> subsystem makes sure that this doesn't happen under normal circumstances?

When TX ring is about to be filler, driver lpc_eth_hard_start_xmit()
calls netif_stop_queue(ndev);

So network stack should not call again lpc_eth_hard_start_xmit().

I would say the bug(s) come from __lpc_handle_xmit(), since it does :

if (netif_queue_stopped(ndev))
	netif_wake_queue(ndev);

without making sure some room is available in TX ring.

cumulative patch :

diff --git a/drivers/net/ethernet/nxp/lpc_eth.c b/drivers/net/ethernet/nxp/lpc_eth.c
index 8d2666f..59b37c8 100644
--- a/drivers/net/ethernet/nxp/lpc_eth.c
+++ b/drivers/net/ethernet/nxp/lpc_eth.c
@@ -946,16 +946,16 @@ static void __lpc_handle_xmit(struct net_device *ndev)
 			/* Update stats */
 			ndev->stats.tx_packets++;
 			ndev->stats.tx_bytes += skb->len;
-
-			/* Free buffer */
-			dev_kfree_skb_irq(skb);
 		}
+		dev_kfree_skb_irq(skb);
 
 		txcidx = readl(LPC_ENET_TXCONSUMEINDEX(pldat->net_base));
 	}
 
-	if (netif_queue_stopped(ndev))
-		netif_wake_queue(ndev);
+	if (pldat->num_used_tx_buffs <= ENET_TX_DESC/2) { 
+		if (netif_queue_stopped(ndev))
+			netif_wake_queue(ndev);
+	}
 }
 
 static int __lpc_handle_recv(struct net_device *ndev, int budget)


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