Unable to update table after creating an After update trigger I have a table like this

/*Table: pageprivilege*/
------------------------

/*Column Information*/
----------------------

FIELD   TYPE              COLLATION          NULL    KEY     DEFAULT  Extra           PRIVILEGES                       COMMENT
------  ----------------  -----------------  ------  ------  -------  --------------  -------------------------------  -------
id      INT(10) UNSIGNED  (NULL)             NO      PRI     (NULL)   AUTO_INCREMENT  SELECT,INSERT,UPDATE,REFERENCES         
pageid  VARCHAR(20)       latin1_swedish_ci  YES             (NULL)                   SELECT,INSERT,UPDATE,REFERENCES         
roleid  TINYINT(4)        (NULL)             YES             (NULL)                   SELECT,INSERT,UPDATE,REFERENCES         
ad      TINYINT(1)        (NULL)             YES             0                        SELECT,INSERT,UPDATE,REFERENCES         
ed      TINYINT(1)        (NULL)             YES             0                        SELECT,INSERT,UPDATE,REFERENCES         
dl      TINYINT(1)        (NULL)             YES             0                        SELECT,INSERT,UPDATE,REFERENCES         
rd      TINYINT(1)        (NULL)             YES             0                        SELECT,INSERT,UPDATE,REFERENCES         
st      TINYINT(1)        (NULL)             YES             0                        SELECT,INSERT,UPDATE,REFERENCES   


I have an condition that, if the rows are updated such that

ad, ed, dl, rd are all set as 0 then the value of st automatically gets changed to 0 too

For that I wrote a procedure 

CREATE
    /*[DEFINER = { user | CURRENT_USER }]*/
    PROCEDURE `smsdev`.`update_priv`()
    /*LANGUAGE SQL
    | [NOT] DETERMINISTIC
    | { CONTAINS SQL | NO SQL | READS SQL DATA | MODIFIES SQL DATA }
    | SQL SECURITY { DEFINER | INVOKER }
    | COMMENT 'string'*/
    BEGIN

     UPDATE pageprivilege SET st=0 WHERE ad = 0 AND ed =0 AND dl =0 AND rd = 0;

    END$$

DELIMITER ;  


And added a trigger which gets called after the table has been updated and calls the procedure like this

DELIMITER $$

USE `smsdev`$$

DROP TRIGGER /*!50032 IF EXISTS */ `pageprivilege_update`$$

CREATE
    /*!50017 DEFINER = 'smsdev'@'%' */
    TRIGGER `pageprivilege_update` AFTER UPDATE ON `pageprivilege` 
    FOR EACH ROW BEGIN
     CALL update_priv();
END;
$$

DELIMITER ;


The issue is whenever I make any update to the any of the 4 field, ad, ed, dl, rd I get this error 


  Can't update table 'pageprivilege' in stored function/trigger because
  it is already used by statement which invoked this stored
  function/trigger.


I am new to using trigger and stored procedure, I did read that is a bad idea to fire procedure from inside trigger, but this is just like a test of idea for me so if you could just bear with me and help me with this hack-ish code it would be gr8.
Thats correct you can not update the same table where the trigger is getting executed, however if you use before update you can set the st to 0 by checking the conditions something as

DELIMITER $$

USE `smsdev`$$

DROP TRIGGER /*!50032 IF EXISTS */ `pageprivilege_update`$$

CREATE
    /*!50017 DEFINER = 'smsdev'@'%' */
    TRIGGER `pageprivilege_update` BEFORE UPDATE ON `pageprivilege` 
    FOR EACH ROW 
    BEGIN
     if (new.ad = 0 AND new.ed =0 AND new.dl =0 AND new.rd = 0) then
       set new.st=0 ;
     end if;
END;
$$

DELIMITER ;