[AF_IUCV]: defensive programming of iucv_callback_txdone

The loop in iucv_callback_txdone presumes existence of an entry
with msg->tag in the send_skb_q list. In error cases this
assumption might be wrong and might cause an endless loop.
Loop is rewritten to guarantee loop end in case of missing
msg->tag entry in send_skb_q.
Signed-off-by: NUrsula Braun <braunu@de.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/iucv/af_iucv.c

@@ -1112,24 +1112,31 @@ static void iucv_callback_txdone(struct iucv_path *path,
 				 struct iucv_message *msg)
 {
 	struct sock *sk = path->private;
-	struct sk_buff *this;
+	struct sk_buff *this = NULL;
 	struct sk_buff_head *list = &iucv_sk(sk)->send_skb_q;
 	struct sk_buff *list_skb = list->next;
 	unsigned long flags;
 
-	if (list_skb) {
+	if (!skb_queue_empty(list)) {
 		spin_lock_irqsave(&list->lock, flags);
 
-		do {
-			this = list_skb;
+		while (list_skb != (struct sk_buff *)list) {
+			if (!memcmp(&msg->tag, list_skb->cb, 4)) {
+				this = list_skb;
+				break;
+			}
 			list_skb = list_skb->next;
-		} while (memcmp(&msg->tag, this->cb, 4) && list_skb);
-		__skb_unlink(this, list);
+		}
+		if (this)
+			__skb_unlink(this, list);
 
 		spin_unlock_irqrestore(&list->lock, flags);
 
-		kfree_skb(this);
+		if (this)
+			kfree_skb(this);
 	}
+	if (!this)
+		printk(KERN_ERR "AF_IUCV msg tag %u not found\n", msg->tag);
 
 	if (sk->sk_state == IUCV_CLOSING) {
 		if (skb_queue_empty(&iucv_sk(sk)->send_skb_q)) {