# HG changeset patch
# User Ian Campbell <ian.campbell@xxxxxxxxxx>
# Date 1206697762 0
# Node ID 0637d22ed554eb0b9cecc7a74fa0a77a7c456497
# Parent  fdb998e79aba45e27948047f680bf70ca5dddbd9
Avoid deadlock when unregistering a xenbus watch.

Watch handlers which run in a separate thread (XBWF_new_thread) should
run without the xenbus_mutex held since kthread_run can block waiting
for memory which causes a deadlock if further watches need to be
unregistered in order to activate the swap device on resume.

XBWF_new_thread cannot be safely unregistered anyway since the mutex
only protects thread startup.

Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>

diff -r fdb998e79aba -r 0637d22ed554 drivers/xen/xenbus/xenbus_xs.c
--- a/drivers/xen/xenbus/xenbus_xs.c	Fri Mar 28 09:44:51 2008 +0000
+++ b/drivers/xen/xenbus/xenbus_xs.c	Fri Mar 28 09:49:22 2008 +0000
@@ -629,6 +629,8 @@ void unregister_xenbus_watch(struct xenb
 	char token[sizeof(watch) * 2 + 1];
 	int err;
 
+	BUG_ON(watch->flags & XBWF_new_thread);
+
 	sprintf(token, "%lX", (long)watch);
 
 	down_read(&xs_state.watch_mutex);
@@ -738,16 +740,29 @@ static int xenwatch_thread(void *unused)
 			list_del(ent);
 		spin_unlock(&watch_events_lock);
 
-		if (ent != &watch_events) {
-			msg = list_entry(ent, struct xs_stored_msg, list);
-			if (msg->u.watch.handle->flags & XBWF_new_thread)
-				kthread_run(xenwatch_handle_callback,
-					    msg, "xenwatch_cb");
-			else
-				xenwatch_handle_callback(msg);
+		if (ent == &watch_events) {
+			mutex_unlock(&xenwatch_mutex);
+			continue;
 		}
 
-		mutex_unlock(&xenwatch_mutex);
+		msg = list_entry(ent, struct xs_stored_msg, list);
+
+		/*
+		 * Unlock the mutex before running an XBWF_new_thread
+		 * handler. kthread_run can block which can deadlock
+		 * against unregister_xenbus_watch() if we need to
+		 * unregister other watches in order to make
+		 * progress. This can occur on resume before the swap
+		 * device is attached.
+		 */
+		if (msg->u.watch.handle->flags & XBWF_new_thread) {
+			mutex_unlock(&xenwatch_mutex);
+			kthread_run(xenwatch_handle_callback,
+				    msg, "xenwatch_cb");
+		} else {
+			xenwatch_handle_callback(msg);
+			mutex_unlock(&xenwatch_mutex);
+		}
 	}
 
 	return 0;