Use module use count on accel plugin to prevent premature unload

diff -r 3e8b5cb457d9 drivers/xen/netback/accel.c
--- a/drivers/xen/netback/accel.c	Wed Oct 03 13:38:09 2007 +0100
+++ b/drivers/xen/netback/accel.c	Wed Oct 03 13:41:15 2007 +0100
@@ -187,8 +187,8 @@ static int netback_accelerator_cleanup_b
  *
  * This should only be allowed when there are no remaining users -
  * i.e. it is not necessary to go through and clear all the hooks, as
- * they should have already been removed.  This is enforced through a
- * usage count and BUG_ON(use!=0), but should be made more user-friendly
+ * they should have already been removed.  This is enforced by taking
+ * a module reference to the plugin while the interfaces are in use
  */
 void netback_disconnect_accelerator(int id, const char *eth_name)
 {
@@ -227,7 +227,8 @@ void netback_probe_accelerators(struct b
 	 */
 	spin_lock_irqsave(&accelerators_lock, flags);
 	list_for_each_entry(accelerator, &accelerators_list, link) { 
-		if (match_accelerator(dev, be, accelerator)) {
+		if (match_accelerator(dev, be, accelerator) &&
+		    try_module_get(accelerator->hooks->owner)) {
 			be->accelerator = accelerator;
 			atomic_inc(&be->accelerator->use_count);
 			be->accelerator->hooks->probe(dev);
@@ -242,9 +243,10 @@ void netback_remove_accelerators(struct 
 				 struct xenbus_device *dev)
 {
 	/* Notify the accelerator (if any) of this device's removal */
-	if ( be->accelerator ) {
+	if (be->accelerator) {
 		be->accelerator->hooks->remove(dev);
 		atomic_dec(&be->accelerator->use_count);
+		module_put(be->accelerator->hooks->owner);
 	}
 	be->accelerator = NULL;
 }
diff -r 3e8b5cb457d9 drivers/xen/netback/common.h
--- a/drivers/xen/netback/common.h	Wed Oct 03 13:38:09 2007 +0100
+++ b/drivers/xen/netback/common.h	Wed Oct 03 13:38:46 2007 +0100
@@ -125,6 +125,7 @@ extern int netbk_copy_skb_mode;
 
 /* Function pointers into netback accelerator plugin modules */
 struct netback_accel_hooks {
+	struct module *owner;
 	int  (*probe)(struct xenbus_device *dev);
 	int (*remove)(struct xenbus_device *dev);
 };