Commit 72e97882 authored by Davi Arnaut's avatar Davi Arnaut

Bug#43230: SELECT ... FOR UPDATE can hang with FLUSH TABLES WITH READ LOCK indefinitely

The problem is that a SELECT .. FOR UPDATE statement might open
a table and later wait for a impeding global read lock without
noticing whether it is holding a table that is being waited upon
the the flush phase of the process that took the global read
lock.

The same problem also affected the following statements:

LOCK TABLES .. WRITE
UPDATE .. SET (update and multi-table update)
TRUNCATE TABLE ..
LOAD DATA ..

The solution is to make the above statements wait for a impending
global read lock before opening the tables. If there is no
impending global read lock, the statement raises a temporary
protection against global read locks and progresses smoothly
towards completion.

Important notice: the patch does not try to address all possible
cases, only those which are common and can be fixed unintrusively
enough for 5.0.

mysql-test/r/lock_multi.result:
  Add test case result for Bug#43230
mysql-test/t/lock_multi.test:
  Add test case for Bug#43230
sql/sql_lex.cc:
  Initialize flag.
sql/sql_lex.h:
  Add a flag to the lexer.
sql/sql_parse.cc:
  Wait for the global read lock is a write lock is going to be
  taken. The wait is done before opening tables.
sql/sql_yacc.yy:
  Protect against the GRL if its a SELECT .. FOR UPDATE or LOCK TABLES
  .. WRITE statement.
parent 02f8ec4d
......@@ -133,3 +133,58 @@ ALTER TABLE t1 ADD COLUMN a INT;
# 2.2.1. normal mode
# 2.2.2. PS mode
DROP TABLE t1;
create table t1 (a int);
create table t2 like t1;
# con1
lock tables t1 write;
# con2
flush tables with read lock;
# con5
# global read lock is taken
# con3
select * from t2 for update;
# waiting for release of read lock
# con4
# would hang and later cause a deadlock
flush tables t2;
# clean up
unlock tables;
unlock tables;
a
drop table t1,t2;
#
# Lightweight version:
# Ensure that the wait for a GRL is done before opening tables.
#
create table t1 (a int);
create table t2 like t1;
#
# UPDATE
#
# default
flush tables with read lock;
# con1
update t2 set a = 1;
# default
# statement is waiting for release of read lock
# con2
flush table t2;
# default
unlock tables;
# con1
#
# LOCK TABLES .. WRITE
#
# default
flush tables with read lock;
# con1
lock tables t2 write;
# default
# statement is waiting for release of read lock
# con2
flush table t2;
# default
unlock tables;
# con1
unlock tables;
drop table t1,t2;
......@@ -683,6 +683,134 @@ DROP TABLE t1;
--disconnect locker
--disconnect writer
#
# Bug#43230: SELECT ... FOR UPDATE can hang with FLUSH TABLES WITH READ LOCK indefinitely
#
connect (con1,localhost,root,,);
connect (con2,localhost,root,,);
connect (con3,localhost,root,,);
connect (con4,localhost,root,,);
connect (con5,localhost,root,,);
create table t1 (a int);
create table t2 like t1;
connection con1;
--echo # con1
lock tables t1 write;
connection con2;
--echo # con2
send flush tables with read lock;
connection con5;
--echo # con5
let $show_statement= SHOW PROCESSLIST;
let $field= State;
let $condition= = 'Flushing tables';
--source include/wait_show_condition.inc
--echo # global read lock is taken
connection con3;
--echo # con3
send select * from t2 for update;
connection con5;
let $show_statement= SHOW PROCESSLIST;
let $field= State;
let $condition= = 'Waiting for release of readlock';
--source include/wait_show_condition.inc
--echo # waiting for release of read lock
connection con4;
--echo # con4
--echo # would hang and later cause a deadlock
flush tables t2;
connection con1;
--echo # clean up
unlock tables;
connection con2;
--reap
unlock tables;
connection con3;
--reap
connection default;
disconnect con5;
disconnect con4;
disconnect con3;
disconnect con2;
disconnect con1;
drop table t1,t2;
--echo #
--echo # Lightweight version:
--echo # Ensure that the wait for a GRL is done before opening tables.
--echo #
connect (con1,localhost,root,,);
connect (con2,localhost,root,,);
create table t1 (a int);
create table t2 like t1;
--echo #
--echo # UPDATE
--echo #
connection default;
--echo # default
flush tables with read lock;
connection con1;
--echo # con1
send update t2 set a = 1;
connection default;
--echo # default
let $show_statement= SHOW PROCESSLIST;
let $field= State;
let $condition= = 'Waiting for release of readlock';
--source include/wait_show_condition.inc
--echo # statement is waiting for release of read lock
connection con2;
--echo # con2
flush table t2;
connection default;
--echo # default
unlock tables;
connection con1;
--echo # con1
--reap
--echo #
--echo # LOCK TABLES .. WRITE
--echo #
connection default;
--echo # default
flush tables with read lock;
connection con1;
--echo # con1
send lock tables t2 write;
connection default;
--echo # default
let $show_statement= SHOW PROCESSLIST;
let $field= State;
let $condition= = 'Waiting for release of readlock';
--source include/wait_show_condition.inc
--echo # statement is waiting for release of read lock
connection con2;
--echo # con2
flush table t2;
connection default;
--echo # default
unlock tables;
connection con1;
--echo # con1
--reap
unlock tables;
connection default;
disconnect con2;
disconnect con1;
drop table t1,t2;
# End of 5.0 tests
# Wait till all disconnects are completed
......
......@@ -204,6 +204,7 @@ void lex_start(THD *thd)
lex->nest_level=0 ;
lex->allow_sum_func= 0;
lex->in_sum_func= NULL;
lex->protect_against_global_read_lock= FALSE;
DBUG_VOID_RETURN;
}
......
......@@ -1176,6 +1176,22 @@ typedef struct st_lex : public Query_tables_list
bool escape_used;
/*
Special case for SELECT .. FOR UPDATE and LOCK TABLES .. WRITE.
Protect from a impending GRL as otherwise the thread might deadlock
if it starts waiting for the GRL in mysql_lock_tables.
The protection is needed because there is a race between setting
the global read lock and waiting for all open tables to be closed.
The problem is a circular wait where a thread holding "old" open
tables will wait for the global read lock to be released while the
thread holding the global read lock will wait for all "old" open
tables to be closed -- the flush part of flush tables with read
lock.
*/
bool protect_against_global_read_lock;
st_lex();
virtual ~st_lex()
......
......@@ -2800,6 +2800,10 @@ mysql_execute_command(THD *thd)
if (res)
goto error;
if (!thd->locked_tables && lex->protect_against_global_read_lock &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
if (!(res= open_and_lock_tables(thd, all_tables)))
{
if (lex->describe)
......@@ -3660,6 +3664,9 @@ end_with_restore_list:
DBUG_ASSERT(first_table == all_tables && first_table != 0);
if (update_precheck(thd, all_tables))
break;
if (!thd->locked_tables &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
DBUG_ASSERT(select_lex->offset_limit == 0);
unit->set_limit(select_lex);
res= (up_result= mysql_update(thd, all_tables,
......@@ -3686,6 +3693,15 @@ end_with_restore_list:
else
res= 0;
/*
Protection might have already been risen if its a fall through
from the SQLCOM_UPDATE case above.
*/
if (!thd->locked_tables &&
lex->sql_command == SQLCOM_UPDATE_MULTI &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
res= mysql_multi_update_prepare(thd);
#ifdef HAVE_REPLICATION
......@@ -3853,7 +3869,8 @@ end_with_restore_list:
ER(ER_LOCK_OR_ACTIVE_TRANSACTION), MYF(0));
goto error;
}
if (!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
res= mysql_truncate(thd, first_table, 0);
break;
case SQLCOM_DELETE:
......@@ -4027,6 +4044,10 @@ end_with_restore_list:
if (check_one_table_access(thd, privilege, all_tables))
goto error;
if (!thd->locked_tables &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
res= mysql_load(thd, lex->exchange, first_table, lex->field_list,
lex->update_list, lex->value_list, lex->duplicates,
lex->ignore, (bool) lex->local_file);
......@@ -4082,6 +4103,9 @@ end_with_restore_list:
goto error;
if (check_table_access(thd, LOCK_TABLES_ACL | SELECT_ACL, all_tables, 0))
goto error;
if (lex->protect_against_global_read_lock &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
goto error;
thd->in_lock_tables=1;
thd->options|= OPTION_TABLE_LOCK;
......
......@@ -4522,6 +4522,7 @@ select_lock_type:
LEX *lex=Lex;
lex->current_select->set_lock_for_tables(TL_WRITE);
lex->safe_to_cache_query=0;
lex->protect_against_global_read_lock= TRUE;
}
| LOCK_SYM IN_SYM SHARE_SYM MODE_SYM
{
......@@ -10058,8 +10059,12 @@ table_lock_list:
table_lock:
table_ident opt_table_alias lock_option
{
if (!Select->add_table_to_list(YYTHD, $1, $2, 0, (thr_lock_type) $3))
thr_lock_type lock_type= (thr_lock_type) $3;
if (!Select->add_table_to_list(YYTHD, $1, $2, 0, lock_type))
MYSQL_YYABORT;
/* If table is to be write locked, protect from a impending GRL. */
if (lock_type >= TL_WRITE_ALLOW_WRITE)
Lex->protect_against_global_read_lock= TRUE;
}
;
......
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