A follow up for the previous patch, titled:

A code review comment for Bug#52289.
Encapsulate the deadlock detection functionality into
a visitor class...

Remove a race introduced by omission: 
initialize iterators under a read lock on the object.

sql/mdl.cc:
  Initialize iterators under a read lock on the object.

sql/mdl.cc

@@ -1854,15 +1854,16 @@ bool MDL_lock::find_deadlock(MDL_ticket *waiting_ticket,
   MDL_ticket *ticket;
   MDL_context *src_ctx= waiting_ticket->get_ctx();
   bool result= TRUE;
-  Ticket_iterator granted_it(m_granted);
-  Ticket_iterator waiting_it(m_waiting);
-
 
   if (dvisitor->enter_node(src_ctx))
     return TRUE;
 
   mysql_prlock_rdlock(&m_rwlock);
 
+  /* Must be initialized after taking a read lock. */
+  Ticket_iterator granted_it(m_granted);
+  Ticket_iterator waiting_it(m_waiting);
+
   /*
     We do a breadth-first search first -- that is, inspect all
     edges of the current node, and only then follow up to the next