spidernet: transmit race

Multiple threads performing a transmit can race into
the spidernet tx ring cleanup code. This puts the
relevant check under a lock.
Signed-off-by: NLinas Vepstas <lins@austin.ibm.com>
Cc: Jens Osterkamp <Jens.Osterkamp@de.ibm.com>
Cc: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/net/spider_net.c

@@ -803,8 +803,12 @@ spider_net_release_tx_chain(struct spider_net_card *card, int brutal)
 	unsigned long flags;
 	int status;
 
-	while (chain->tail != chain->head) {
+	while (1) {
 		spin_lock_irqsave(&chain->lock, flags);
+		if (chain->tail == chain->head) {
+			spin_unlock_irqrestore(&chain->lock, flags);
+			return 0;
+		}
 		descr = chain->tail;
 		hwdescr = descr->hwdescr;