xenbus: add missing wakeup in concurrent read/write

If an application has a dedicated read thread watching xenbus and
another thread writes an XS_WATCH message that generates a synthetic
"OK" reply, this reply will be enqueued in the buffer without waking up
the reader. This can cause a deadlock in the application if it then
waits for the read thread to receive the queued message.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>

commit e752969f502a511e83f841aa01d6cd332e6d85a0
Author: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Date:   Tue Sep 7 11:21:52 2010 -0400

    xenbus: fix deadlock in concurrent read/write

    If an application has a dedicated read thread watching xenbus and another
    thread writes an XS_WATCH message that generates a synthetic "OK" reply,
    this reply will be enqueued in the buffer without waking up the reader.
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/xenfs/xenbus.c

@@ -405,6 +405,7 @@ static int xenbus_write_watch(unsigned msg_type, struct xenbus_file_priv *u)
 
 		mutex_lock(&u->reply_mutex);
 		rc = queue_reply(&u->read_buffers, &reply, sizeof(reply));
+		wake_up(&u->read_waitq);
 		mutex_unlock(&u->reply_mutex);
 	}