sql_db.cc:

  On Monty's suggestion move ha_drop_database back inside LOCK_mysql_create_db, but we release LOCK_open during it to let possible running queries complete. I tested this and it seemed to work also in complex concurrent DELETE, SELECT, DROP DATABASE, and CREATE DATABASE situations

sql/sql_db.cc

@@ -166,6 +166,15 @@ void mysql_rm_db(THD *thd,char *db,bool if_exists)
 
   if ((deleted=mysql_rm_known_files(thd, dirp, path,0)) >= 0)
   {
+    /* If there are running queries on the tables, MySQL needs to get
+       access to LOCK_open to end them. InnoDB on the other hand waits
+       for the queries to end before dropping the database. That is why we
+       must do the dropping with LOCK_open released. */
+
+    VOID(pthread_mutex_unlock(&LOCK_open));
+    ha_drop_database(path);
+    VOID(pthread_mutex_lock(&LOCK_open));
+
     if (!thd->query)
     {
       thd->query = path;
@@ -190,16 +199,6 @@ exit:
   VOID(pthread_mutex_unlock(&LOCK_open));
   VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
 
-  /* If there are running queries on the tables, MySQL needs to get
-     access to LOCK_open to end them. InnoDB on the other hand waits
-     for the queries to end before dropping the database. That is why we
-     must do the dropping outside of the mutexes above, otherwise the server
-     always hangs if there are running queries. We only drop inside InnoDB
-     if deleted got value >= 0 which means that it was successful. */
-
-  if (deleted >= 0)
-    ha_drop_database(path);
-
   DBUG_VOID_RETURN;
 }