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Bug#19815 - CREATE/RENAME/DROP DATABASE can deadlock on a global read lock

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The order of acquiring LOCK_mysql_create_db
and wait_if_global_read_lock() was wrong. It could happen
that a thread held LOCK_mysql_create_db while waiting for
the global read lock to be released. The thread with the
global read lock could try to administrate a database too.
It would first try to lock LOCK_mysql_create_db and hang...

The check if the current thread has the global read lock
is done in wait_if_global_read_lock(), which could not be
reached because of the hang in LOCK_mysql_create_db.

Now I exchanged the order of acquiring LOCK_mysql_create_db
and wait_if_global_read_lock(). This makes 
wait_if_global_read_lock() fail with an error message for
the thread with the global read lock. No deadlock happens.
This commit is contained in:
ingo@mysql.com
2006-05-29 15:26:23 +02:00
parent da1fdb8a5f
commit d00441e328
3 changed files with 89 additions and 14 deletions
Download Patch File Download Diff File Expand all files Collapse all files

8
mysql-test/r/lock_multi.result
View File

@@ -43,3 +43,11 @@ Field Type Null Key Default Extra
a int(11) YES NULL
unlock tables;
drop table t1;
CREATE DATABASE mysqltest_1;
FLUSH TABLES WITH READ LOCK;
DROP DATABASE mysqltest_1;
DROP DATABASE mysqltest_1;
ERROR HY000: Can't execute the query because you have a conflicting read lock
UNLOCK TABLES;
DROP DATABASE mysqltest_1;
ERROR HY000: Can't drop database 'mysqltest_1'; database doesn't exist

35
mysql-test/t/lock_multi.test
View File

@@ -107,3 +107,38 @@ show columns from t1;
connection locker;
unlock tables;
drop table t1;
#
# Bug#19815 - CREATE/RENAME/DROP DATABASE can deadlock on a global read lock
#
connect (con1,localhost,root,,);
connect (con2,localhost,root,,);
#
connection con1;
CREATE DATABASE mysqltest_1;
FLUSH TABLES WITH READ LOCK;
#
# With bug in place: acquire LOCK_mysql_create_table and
# wait in wait_if_global_read_lock().
connection con2;
send DROP DATABASE mysqltest_1;
--sleep 1
#
# With bug in place: try to acquire LOCK_mysql_create_table...
# When fixed: Reject dropping db because of the read lock.
connection con1;
--error ER_CANT_UPDATE_WITH_READLOCK
DROP DATABASE mysqltest_1;
UNLOCK TABLES;
#
connection con2;
reap;
#
connection default;
disconnect con1;
disconnect con2;
# This must have been dropped by connection 2 already,
# which waited until the global read lock was released.
--error ER_DB_DROP_EXISTS
DROP DATABASE mysqltest_1;

60
sql/sql_db.cc
View File

@@ -425,15 +425,26 @@ bool mysql_create_db(THD *thd, char *db, HA_CREATE_INFO *create_info,
DBUG_RETURN(-1);
}
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
/* do not create database if another thread is holding read lock */
/*
Do not create database if another thread is holding read lock.
Wait for global read lock before acquiring LOCK_mysql_create_db.
After wait_if_global_read_lock() we have protection against another
global read lock. If we would acquire LOCK_mysql_create_db first,
another thread could step in and get the global read lock before we
reach wait_if_global_read_lock(). If this thread tries the same as we
(admin a db), it would then go and wait on LOCK_mysql_create_db...
Furthermore wait_if_global_read_lock() checks if the current thread
has the global read lock and refuses the operation with
ER_CANT_UPDATE_WITH_READLOCK if applicable.
*/
if (wait_if_global_read_lock(thd, 0, 1))
{
error= -1;
goto exit2;
}
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
/* Check directory */
strxmov(path, mysql_data_home, "/", db, NullS);
path_len= unpack_dirname(path,path); // Convert if not unix
@@ -537,9 +548,9 @@ bool mysql_create_db(THD *thd, char *db, HA_CREATE_INFO *create_info,
}
exit:
VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
start_waiting_global_read_lock(thd);
exit2:
VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
DBUG_RETURN(error);
}
@@ -553,12 +564,23 @@ bool mysql_alter_db(THD *thd, const char *db, HA_CREATE_INFO *create_info)
int error= 0;
DBUG_ENTER("mysql_alter_db");
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
/* do not alter database if another thread is holding read lock */
/*
Do not alter database if another thread is holding read lock.
Wait for global read lock before acquiring LOCK_mysql_create_db.
After wait_if_global_read_lock() we have protection against another
global read lock. If we would acquire LOCK_mysql_create_db first,
another thread could step in and get the global read lock before we
reach wait_if_global_read_lock(). If this thread tries the same as we
(admin a db), it would then go and wait on LOCK_mysql_create_db...
Furthermore wait_if_global_read_lock() checks if the current thread
has the global read lock and refuses the operation with
ER_CANT_UPDATE_WITH_READLOCK if applicable.
*/
if ((error=wait_if_global_read_lock(thd,0,1)))
goto exit2;
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
/* Check directory */
strxmov(path, mysql_data_home, "/", db, "/", MY_DB_OPT_FILE, NullS);
fn_format(path, path, "", "", MYF(MY_UNPACK_FILENAME));
@@ -596,9 +618,9 @@ bool mysql_alter_db(THD *thd, const char *db, HA_CREATE_INFO *create_info)
send_ok(thd, result);
exit:
VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
start_waiting_global_read_lock(thd);
exit2:
VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
DBUG_RETURN(error);
}
@@ -630,15 +652,26 @@ bool mysql_rm_db(THD *thd,char *db,bool if_exists, bool silent)
TABLE_LIST* dropped_tables= 0;
DBUG_ENTER("mysql_rm_db");
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
/* do not drop database if another thread is holding read lock */
/*
Do not drop database if another thread is holding read lock.
Wait for global read lock before acquiring LOCK_mysql_create_db.
After wait_if_global_read_lock() we have protection against another
global read lock. If we would acquire LOCK_mysql_create_db first,
another thread could step in and get the global read lock before we
reach wait_if_global_read_lock(). If this thread tries the same as we
(admin a db), it would then go and wait on LOCK_mysql_create_db...
Furthermore wait_if_global_read_lock() checks if the current thread
has the global read lock and refuses the operation with
ER_CANT_UPDATE_WITH_READLOCK if applicable.
*/
if (wait_if_global_read_lock(thd, 0, 1))
{
error= -1;
goto exit2;
}
VOID(pthread_mutex_lock(&LOCK_mysql_create_db));
(void) sprintf(path,"%s/%s",mysql_data_home,db);
length= unpack_dirname(path,path); // Convert if not unix
strmov(path+length, MY_DB_OPT_FILE); // Append db option file name
@@ -747,7 +780,6 @@ bool mysql_rm_db(THD *thd,char *db,bool if_exists, bool silent)
exit:
(void)sp_drop_db_routines(thd, db); /* QQ Ignore errors for now */
start_waiting_global_read_lock(thd);
/*
If this database was the client's selected database, we silently change the
client's selected database to nothing (to have an empty SELECT DATABASE()
@@ -776,9 +808,9 @@ exit:
thd->db= 0;
thd->db_length= 0;
}
exit2:
VOID(pthread_mutex_unlock(&LOCK_mysql_create_db));
start_waiting_global_read_lock(thd);
exit2:
DBUG_RETURN(error);
}