[PATCH] preempt-safe net/ code

This fixes three locations in net/ where per-CPU data could bite us
under preemption.  This is the result of an audit I did and should
constitute all of the unsafe code in net/.

In net/core/skbuff.c I did not have to introduce any code - just
rearrange the grabbing of smp_processor_id() to be in the interrupt off
region.  Pretty clean fixes.

Note in the future we can use put_cpu() and get_cpu() to grab the CPU#
safely.  I will send a patch to Marcelo so we can have a 2.4 version
(which doesn't do the preempt stuff), too...

net/core/dev.c

@@ -1047,9 +1047,15 @@ int dev_queue_xmit(struct sk_buff *skb)
 		int cpu = smp_processor_id();
 
 		if (dev->xmit_lock_owner != cpu) {
+			/*
+			 * The spin_lock effectivly does a preempt lock, but 
+			 * we are about to drop that...
+			 */
+			preempt_disable();
 			spin_unlock(&dev->queue_lock);
 			spin_lock(&dev->xmit_lock);
 			dev->xmit_lock_owner = cpu;
+			preempt_enable();
 
 			if (!netif_queue_stopped(dev)) {
 				if (netdev_nit)

net/core/skbuff.c

@@ -111,33 +111,37 @@ void skb_under_panic(struct sk_buff *skb, int sz, void *here)
 
 static __inline__ struct sk_buff *skb_head_from_pool(void)
 {
-	struct sk_buff_head *list = &skb_head_pool[smp_processor_id()].list;
-
-	if (skb_queue_len(list)) {
-		struct sk_buff *skb;
+	struct sk_buff_head *list;
+	struct sk_buff *skb = NULL;
 	unsigned long flags;
 
 	local_irq_save(flags);
+
+	list = &skb_head_pool[smp_processor_id()].list;
+
+	if (skb_queue_len(list))
 		skb = __skb_dequeue(list);
+
 	local_irq_restore(flags);
 	return skb;
-	}
-	return NULL;
 }
 
 static __inline__ void skb_head_to_pool(struct sk_buff *skb)
 {
-	struct sk_buff_head *list = &skb_head_pool[smp_processor_id()].list;
-
-	if (skb_queue_len(list) < sysctl_hot_list_len) {
+	struct sk_buff_head *list;
 	unsigned long flags;
 
 	local_irq_save(flags);
+	list = &skb_head_pool[smp_processor_id()].list;
+
+	if (skb_queue_len(list) < sysctl_hot_list_len) {
 		__skb_queue_head(list, skb);
 		local_irq_restore(flags);
 
 		return;
 	}
+
+	local_irq_restore(flags);
 	kmem_cache_free(skbuff_head_cache, skb);
 }