MDEV-6774 - Deadlock between SELECT, DROP TABLE, SHOW STATUS and

            SET @@global.log_output

Deadlock chain:
rdlock(LOCK_logger) -> lock(LOCK_open)     SELECT 1
lock(LOCK_open)     -> lock(LOCK_status)   DROP TABLE t1
lock(LOCK_status)   -> lock(LOCK_g_s_v)    SHOW STATUS
lock(LOCK_g_s_)     -> wrlock(LOCK_logger) SET @@global.log_output=DEFAULT

Fixed by removing relationship between LOCK_status and
LOCK_global_system_variables during SHOW STATUS: we don't really need
LOCK_global_system_variables when accessing status vars.

sql/event_scheduler.cc

@@ -355,14 +355,7 @@ Event_scheduler::Event_scheduler(Event_queue *queue_arg)
   mysql_mutex_init(key_event_scheduler_LOCK_scheduler_state,
                    &LOCK_scheduler_state, MY_MUTEX_INIT_FAST);
   mysql_cond_init(key_event_scheduler_COND_state, &COND_state, NULL);
-
-#ifdef SAFE_MUTEX
-  /* Ensure right mutex order */
-  mysql_mutex_lock(&LOCK_scheduler_state);
-  mysql_mutex_lock(&LOCK_global_system_variables);
-  mysql_mutex_unlock(&LOCK_global_system_variables);
-  mysql_mutex_unlock(&LOCK_scheduler_state);
-#endif
+  mysql_mutex_record_order(&LOCK_scheduler_state, &LOCK_global_system_variables);
 }
 
 

sql/sql_show.cc

@@ -2648,12 +2648,11 @@ static bool show_status_array(THD *thd, const char *wild,
         char *value=var->value;
         const char *pos, *end;                  // We assign a lot of const's
 
-        mysql_mutex_lock(&LOCK_global_system_variables);
-
         if (show_type == SHOW_SYS)
         {
           sys_var *var= ((sys_var *) value);
           show_type= var->show_type();
+          mysql_mutex_lock(&LOCK_global_system_variables);
           value= (char*) var->value_ptr(thd, value_type, &null_lex_str);
           charset= var->charset(thd);
         }
@@ -2754,7 +2753,8 @@ static bool show_status_array(THD *thd, const char *wild,
         thd->count_cuted_fields= CHECK_FIELD_IGNORE;
         table->field[1]->set_notnull();
 
-        mysql_mutex_unlock(&LOCK_global_system_variables);
+        if (var->type == SHOW_SYS)
+          mysql_mutex_unlock(&LOCK_global_system_variables);
 
         if (schema_table_store_record(thd, table))
         {