xen/xenbus: Count pending messages for each watch

This commit adds a counter of pending messages for each watch in the
struct.  It is used to skip unnecessary pending messages lookup in
'unregister_xenbus_watch()'.  It could also be used in 'will_handle'
callback.

This is part of XSA-349

Cc: stable@vger.kernel.org
Signed-off-by: SeongJae Park <sjpark@amazon.de>
Reported-by: Michael Kurth <mku@amazon.de>
Reported-by: Pawel Wieczorkiewicz <wipawel@amazon.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/xenbus_xs.c

@@ -711,6 +711,7 @@ int xs_watch_msg(struct xs_watch_event *event)
 				 event->path, event->token))) {
 		spin_lock(&watch_events_lock);
 		list_add_tail(&event->list, &watch_events);
+		event->handle->nr_pending++;
 		wake_up(&watch_events_waitq);
 		spin_unlock(&watch_events_lock);
 	} else
@@ -768,6 +769,8 @@ int register_xenbus_watch(struct xenbus_watch *watch)
 
 	sprintf(token, "%lX", (long)watch);
 
+	watch->nr_pending = 0;
+
 	down_read(&xs_watch_rwsem);
 
 	spin_lock(&watches_lock);
@@ -817,12 +820,15 @@ void unregister_xenbus_watch(struct xenbus_watch *watch)
 
 	/* Cancel pending watch events. */
 	spin_lock(&watch_events_lock);
+	if (watch->nr_pending) {
 		list_for_each_entry_safe(event, tmp, &watch_events, list) {
 			if (event->handle != watch)
 				continue;
 			list_del(&event->list);
 			kfree(event);
 		}
+		watch->nr_pending = 0;
+	}
 	spin_unlock(&watch_events_lock);
 
 	if (current->pid != xenwatch_pid)
@@ -868,7 +874,6 @@ void xs_suspend_cancel(void)
 
 static int xenwatch_thread(void *unused)
 {
-	struct list_head *ent;
 	struct xs_watch_event *event;
 
 	xenwatch_pid = current->pid;
@@ -883,13 +888,15 @@ static int xenwatch_thread(void *unused)
 		mutex_lock(&xenwatch_mutex);
 
 		spin_lock(&watch_events_lock);
-		ent = watch_events.next;
-		if (ent != &watch_events)
-			list_del(ent);
+		event = list_first_entry_or_null(&watch_events,
+				struct xs_watch_event, list);
+		if (event) {
+			list_del(&event->list);
+			event->handle->nr_pending--;
+		}
 		spin_unlock(&watch_events_lock);
 
-		if (ent != &watch_events) {
-			event = list_entry(ent, struct xs_watch_event, list);
+		if (event) {
 			event->handle->callback(event->handle, event->path,
 						event->token);
 			kfree(event);

include/xen/xenbus.h

@@ -61,6 +61,8 @@ struct xenbus_watch
 	/* Path being watched. */
 	const char *node;
 
+	unsigned int nr_pending;
+
 	/*
 	 * Called just before enqueing new event while a spinlock is held.
 	 * The event will be discarded if this callback returns false.