Skip to content

M
MariaDB
Commit c7c1f219 authored by Jon Olav Hauglid's avatar Jon Olav Hauglid
Browse files
Options

Bug #51160 Deadlock around SET GLOBAL EVENT_SCHEDULER = ON|OFF 

This deadlock could occour betweeen one connection executing
SET GLOBAL EVENT_SCHEDULER= ON and another executing SET GLOBAL
EVENT_SCHEDULER= OFF. The bug was introduced by WL#4738.

The first connection would hold LOCK_event_metadata (protecting
the global variable) while trying to lock LOCK_global_system_variables
starting the event scheduler thread (in THD:init()).

The second connection would hold LOCK_global_system_variables
while trying to get LOCK_event_scheduler after stopping the event
scheduler inside event_scheduler_update().

This patch fixes the problem by not using LOCK_event_metadata to
protect the event_scheduler variable. It is still protected using
LOCK_global_system_variables. This fixes the deadlock as it removes 
one of the two mutexes used to produce it.

However, this patch opens up the possibility that the event_scheduler
variable and the real event_scheduler state can become out of sync
(e.g. variable = OFF, but scheduler running). But this can only
happen under very unlikely conditions - two concurrent SET GLOBAL
statments, with one thread interrupted at the exact wrong moment.
This is preferable to having the possibility of a deadlock.

This patch also fixes a bug where it was possible to exit create_event()
without releasing LOCK_event_metadata if running out of memory during
its exection.

No test case added since a repeatable test case would have required
excessive use of new sync points. Instead we rely on the fact that
this bug was easily reproduceable using RGQ tests.
parent f007796b
Hide whitespace changes
Inline Side-by-side
Showing with 34 additions and 25 deletions
sql/events.cc
View file @ c7c1f219
/* Copyright (C) 2004-2006 MySQL AB, 2008-2009 Sun Microsystems, Inc
/* Copyright (c) 2004, 2010, Oracle and/or its affiliates. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
......@@ -11,7 +11,7 @@
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
#include "mysql_priv.h"
#include "events.h"
......@@ -367,15 +367,14 @@ Events::create_event(THD *thd, Event_parse_data *parse_data,
{
sql_print_error("Event Error: An error occurred while creating query string, "
"before writing it into binary log.");
/* Restore the state of binlog format */
DBUG_ASSERT(!thd->is_current_stmt_binlog_format_row());
if (save_binlog_row_based)
thd->set_current_stmt_binlog_format_row();
DBUG_RETURN(TRUE);
ret= TRUE;
}
else
{
/* If the definer is not set or set to CURRENT_USER, the value of CURRENT_USER
will be written into the binary log as the definer for the SQL thread. */
ret= write_bin_log(thd, TRUE, log_query.c_ptr(), log_query.length());
}
/* If the definer is not set or set to CURRENT_USER, the value of CURRENT_USER
will be written into the binary log as the definer for the SQL thread. */
ret= write_bin_log(thd, TRUE, log_query.c_ptr(), log_query.length());
}
}
mysql_mutex_unlock(&LOCK_event_metadata);
......@@ -1017,7 +1016,11 @@ Events::dump_internal_status()
puts("LLA = Last Locked At LUA = Last Unlocked At");
puts("WOC = Waiting On Condition DL = Data Locked");
mysql_mutex_lock(&LOCK_event_metadata);
/*
opt_event_scheduler should only be accessed while
holding LOCK_global_system_variables.
*/
mysql_mutex_lock(&LOCK_global_system_variables);
if (opt_event_scheduler == EVENTS_DISABLED)
puts("The Event Scheduler is disabled");
else
......@@ -1026,7 +1029,7 @@ Events::dump_internal_status()
event_queue->dump_internal_status();
}
mysql_mutex_unlock(&LOCK_event_metadata);
mysql_mutex_unlock(&LOCK_global_system_variables);
DBUG_VOID_RETURN;
}
......
sql/events.h
View file @ c7c1f219
#ifndef _EVENT_H_
#define _EVENT_H_
/* Copyright (C) 2004-2006 MySQL AB, 2008-2009 Sun Microsystems, Inc
/* Copyright (c) 2004, 2010, Oracle and/or its affiliates. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
......@@ -13,7 +13,7 @@
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/**
@defgroup Event_Scheduler Event Scheduler
......@@ -83,6 +83,7 @@ class Events
See sys_var.cc
*/
enum enum_opt_event_scheduler { EVENTS_OFF, EVENTS_ON, EVENTS_DISABLED };
/* Protected using LOCK_global_system_variables only. */
static uint opt_event_scheduler;
static mysql_mutex_t LOCK_event_metadata;
static bool check_if_system_tables_error();
......@@ -106,9 +107,6 @@ class Events
static void
destroy_mutexes();
static bool
switch_event_scheduler_state(enum enum_opt_event_scheduler new_state);
static bool
create_event(THD *thd, Event_parse_data *parse_data, bool if_exists);
......
sql/sys_vars.cc
View file @ c7c1f219
/* Copyright (C) 2002-2006 MySQL AB, 2009-2010 Sun Microsystems, Inc.
/* Copyright (c) 2002, 2010, Oracle and/or its affiliates. All rights reserved.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
......@@ -11,7 +11,7 @@
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA */
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA */
/*
How to add new variables:
......@@ -647,32 +647,40 @@ static bool event_scheduler_check(sys_var *self, THD *thd, set_var *var)
}
static bool event_scheduler_update(sys_var *self, THD *thd, enum_var_type type)
{
uint opt_event_scheduler_value= Events::opt_event_scheduler;
mysql_mutex_unlock(&LOCK_global_system_variables);
/*
Events::start() is heavyweight. In particular it creates a new THD,
which takes LOCK_global_system_variables internally.
Thus we have to release it here.
We need to re-take it before returning, though.
And we need to take it *without* holding Events::LOCK_event_metadata.
Note that since we release LOCK_global_system_variables before calling
start/stop, there is a possibility that the server variable
can become out of sync with the real event scheduler state.
This can happen with two concurrent statments if the first gets
interrupted after start/stop but before retaking
LOCK_global_system_variables. However, this problem should be quite
rare and it's difficult to avoid it without opening up possibilities
for deadlocks. See bug#51160.
*/
bool ret= Events::opt_event_scheduler == Events::EVENTS_ON
bool ret= opt_event_scheduler_value == Events::EVENTS_ON
? Events::start()
: Events::stop();
mysql_mutex_unlock(&Events::LOCK_event_metadata);
mysql_mutex_lock(&LOCK_global_system_variables);
mysql_mutex_lock(&Events::LOCK_event_metadata);
if (ret)
my_error(ER_EVENT_SET_VAR_ERROR, MYF(0));
return ret;
}
static PolyLock_mutex PLock_event_metadata(&Events::LOCK_event_metadata);
static Sys_var_enum Sys_event_scheduler(
"event_scheduler", "Enable the event scheduler. Possible values are "
"ON, OFF, and DISABLED (keep the event scheduler completely "
"deactivated, it cannot be activated run-time)",
GLOBAL_VAR(Events::opt_event_scheduler), CMD_LINE(OPT_ARG),
event_scheduler_names, DEFAULT(Events::EVENTS_OFF),
&PLock_event_metadata, NOT_IN_BINLOG,
NO_MUTEX_GUARD, NOT_IN_BINLOG,
ON_CHECK(event_scheduler_check), ON_UPDATE(event_scheduler_update));
#endif
......
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment
GitLab Nexedi Edition | About GitLab | About Nexedi | 沪ICP备2021021310号-2 | 沪ICP备2021021310号-7