[0.2-stable]My session never expired

[wolfgao]

the session in roundcube never expired after logged in, nothing happened after the expire time. Next is my setting in main.inc.php.

$rcmail_config['keep_alive'] = 300;          
$rcmail_config['min_keep_alive'] = 60;        
$rcmail_config['session_lifetime'] = 10;      

Is there something wrong in the setting? What should i do with it?