[PATCH 2/2]: Spider ethernet driver -- protect chain head

[linas@austin.ibm.com (Linas Vepstas)]

Please review/apply/ack/forward upstream.

--linas


Prevent a potential race. If two threads are both calling
the transmit routine, both can potentially try to grab the
same dma descriptor. Serialize access to the head of the
tx ring with spinlocks.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>

----
 drivers/net/spider_net.c |   12 ++++++++----
 1 files changed, 8 insertions(+), 4 deletions(-)

Index: linux-2.6.17-rc1/drivers/net/spider_net.c
===================================================================
--- linux-2.6.17-rc1.orig/drivers/net/spider_net.c	2006-04-21 18:32:46.000000000 -0500
+++ linux-2.6.17-rc1/drivers/net/spider_net.c	2006-04-21 18:39:12.000000000 -0500
@@ -806,13 +806,19 @@ spider_net_stop(struct net_device *netde
 static struct spider_net_descr *
 spider_net_get_next_tx_descr(struct spider_net_card *card)
 {
+	struct spider_net_descr *descr;
+	spin_lock(&card->tx_chain_lock);
+
+	descr = card->tx_chain.head;
 	/* check, if head points to not-in-use descr */
 	if ( spider_net_get_descr_status(card->tx_chain.head) ==
 	     SPIDER_NET_DESCR_NOT_IN_USE ) {
-		return card->tx_chain.head;
+		card->tx_chain.head = descr->next;
 	} else {
-		return NULL;
+		descr = NULL;
 	}
+	spin_unlock(&card->tx_chain_lock);
+	return descr;
 }
 
 /**
@@ -932,8 +938,6 @@ spider_net_xmit(struct sk_buff *skb, str
 	if (result)
 		goto error;
 
-	card->tx_chain.head = card->tx_chain.head->next;
-
 	if (spider_net_get_descr_status(descr->prev) !=
 	    SPIDER_NET_DESCR_CARDOWNED) {
 		/* make sure the current descriptor is in memory. Then