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Bug#23713 LOCK TABLES + CREATE TRIGGER + FLUSH TABLES WITH READ LOCK = deadlock

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This bug is actually two bugs in one, one of which is CREATE TRIGGER under
LOCK TABLES and the other is CREATE TRIGGER under LOCK TABLES simultaneous
to a FLUSH TABLES WITH READ LOCK (global read lock). Both situations could
lead to a server crash or deadlock.

The first problem arises from the fact that when under LOCK TABLES, if the
table is in the set of locked tables, the table is already open and it doesn't
need to be reopened (not a placeholder). Also in this case, if the table is
not write locked, a exclusive lock can't be acquired because of a possible
deadlock with another thread also holding a (read) lock on the table. The
second issue arises from the fact that one should never wait for a global
read lock if it's holding any locked tables, because the global read lock
is waiting for these tables and this leads to a circular wait deadlock.

The solution for the first case is to check if the table is write locked
and upgraded the write lock to a exclusive lock and fail otherwise for non
write locked tables. Grabbin the exclusive lock in this case also means
to ensure that the table is opened only by the calling thread. The second
issue is partly fixed by not waiting for the global read lock if the thread
is holding any locked tables.

The second issue is only partly addressed in this patch because it turned
out to be much wider and also affects other DDL statements. Reported as
Bug#32395
This commit is contained in:
davi@mysql.com/endora.local
2007-11-29 09:42:26 -02:00
parent de0175204d
commit d179bb64c2
10 changed files with 207 additions and 47 deletions
Download Patch File Download Diff File Expand all files Collapse all files

68
sql/sql_trigger.cc
View File

@ -323,6 +323,7 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
TABLE *table;
bool result= TRUE;
String stmt_query;
bool need_start_waiting= FALSE;
DBUG_ENTER("mysql_create_or_drop_trigger");
@ -374,10 +375,12 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
/*
We don't want perform our operations while global read lock is held
so we have to wait until its end and then prevent it from occurring
again until we are done. (Acquiring LOCK_open is not enough because
global read lock is held without holding LOCK_open).
again until we are done, unless we are under lock tables. (Acquiring
LOCK_open is not enough because global read lock is held without holding
LOCK_open).
*/
if (wait_if_global_read_lock(thd, 0, 1))
if (!thd->locked_tables &&
!(need_start_waiting= !wait_if_global_read_lock(thd, 0, 1)))
DBUG_RETURN(TRUE);
VOID(pthread_mutex_lock(&LOCK_open));
@ -433,35 +436,25 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
goto end;
}
if (lock_table_names(thd, tables))
goto end;
/* We also don't allow creation of triggers on views. */
tables->required_type= FRMTYPE_TABLE;
/*
If the table is under LOCK TABLES, lock_table_names() does not set
tables->table. Find the table in open_tables.
*/
if (!tables->table && thd->locked_tables)
/* Keep consistent with respect to other DDL statements */
mysql_ha_rm_tables(thd, tables, TRUE);
if (thd->locked_tables)
{
for (table= thd->open_tables;
table && (strcmp(table->s->table_name.str, tables->table_name) ||
strcmp(table->s->db.str, tables->db));
table= table->next) {}
tables->table= table;
/* Table must be write locked */
if (name_lock_locked_table(thd, tables))
goto end;
}
if (!tables->table)
else
{
/* purecov: begin inspected */
my_error(ER_TABLE_NOT_LOCKED, MYF(0), tables->alias);
goto end;
/* purecov: end */
}
/* No need to reopen the table if it is locked with LOCK TABLES. */
if (!thd->locked_tables || (tables->table->in_use != thd))
{
/* We also don't allow creation of triggers on views. */
tables->required_type= FRMTYPE_TABLE;
/* Grab the name lock and insert the placeholder*/
if (lock_table_names(thd, tables))
goto end;
/* Convert the placeholder to a real table */
if (reopen_name_locked_table(thd, tables, TRUE))
{
unlock_table_name(thd, tables);
@ -489,13 +482,20 @@ bool mysql_create_or_drop_trigger(THD *thd, TABLE_LIST *tables, bool create)
/* Under LOCK TABLES we must reopen the table to activate the trigger. */
if (!result && thd->locked_tables)
{
/*
Must not use table->s->db.str or table->s->table_name.str here.
The strings are used in a loop even after the share may be freed.
*/
/* Make table suitable for reopening */
close_data_files_and_morph_locks(thd, tables->db, tables->table_name);
thd->in_lock_tables= 1;
result= reopen_tables(thd, 1, 0);
if (reopen_tables(thd, 1, 1))
{
/* To be safe remove this table from the set of LOCKED TABLES */
unlink_open_table(thd, tables->table, FALSE);
/*
Ignore reopen_tables errors for now. It's better not leave master/slave
in a inconsistent state.
*/
thd->clear_error();
}
thd->in_lock_tables= 0;
}
@ -507,7 +507,9 @@ end:
}
VOID(pthread_mutex_unlock(&LOCK_open));
start_waiting_global_read_lock(thd);
if (need_start_waiting)
start_waiting_global_read_lock(thd);
if (!result)
send_ok(thd);