mv643xx_eth: Fix race condition in mv643xx_eth_free_tx_descs

mv643xx_eth: Fix race condition in mv643xx_eth_free_tx_descs

This bug was found and isolated by Thibaut VARENE <T-Bone@parisc-linux.org>
and Jarek Poplawski <jarkao2@o2.pl>.  This patch is a modification of their
fixes.  We acquire and release the lock for each descriptor that is freed
to minimize the time the lock is held.
Signed-off-by: Jeff Garzik <jeff@garzik.org>

drivers/net/mv643xx_eth.c

@@ -314,6 +314,13 @@ int mv643xx_eth_free_tx_descs(struct net_device *dev, int force)
 
 	while (mp->tx_desc_count > 0) {
 		spin_lock_irqsave(&mp->lock, flags);
+
+		/* tx_desc_count might have changed before acquiring the lock */
+		if (mp->tx_desc_count <= 0) {
+			spin_unlock_irqrestore(&mp->lock, flags);
+			return released;
+		}
+
 		tx_index = mp->tx_used_desc_q;
 		desc = &mp->p_tx_desc_area[tx_index];
 		cmd_sts = desc->cmd_sts;
@@ -332,13 +339,13 @@ int mv643xx_eth_free_tx_descs(struct net_device *dev, int force)
 		if (skb)
 			mp->tx_skb[tx_index] = NULL;
 
-		spin_unlock_irqrestore(&mp->lock, flags);
-
 		if (cmd_sts & ETH_ERROR_SUMMARY) {
 			printk("%s: Error in TX\n", dev->name);
 			mp->stats.tx_errors++;
 		}
 
+		spin_unlock_irqrestore(&mp->lock, flags);
+
 		if (cmd_sts & ETH_TX_FIRST_DESC)
 			dma_unmap_single(NULL, addr, count, DMA_TO_DEVICE);
 		else