Erreur SQL 1442, Requete UPDATE ne marche pas

delimiter //
CREATE TRIGGER TTC_PROMO_UPDATE
        BEFORE UPDATE ON produit
                FOR EACH ROW
                        BEGIN
                                        UPDATE produit,promotion
                                                SET produit.prix_ttc_p = ((produit.prix_ht_p*(100 - promotion.taux)/100)*(1+(produit.tva_p/100)) )
                                                        WHERE promotion.id_promo = produit.id_promo;
                        END;
                        //
delimiter ;
 
 
Erreur : ERROR 1442 (HY000): Can't update table 'produit' in stored function/trigger because it is already used by statement which invoked this stored function/trigger. 
 
mysql> DESC produit;
+---------------+--------------+------+-----+---------+----------------+
| FIELD         | TYPE         | NULL | KEY | DEFAULT | Extra          |
+---------------+--------------+------+-----+---------+----------------+
| id_p          | INT(11)      | NO   | PRI | NULL    | AUTO_INCREMENT |
| ref_p         | INT(11)      | NO   |     | NULL    |                |
| nom_p         | VARCHAR(100) | YES  |     | NULL    |                |
| tva_p         | FLOAT        | YES  |     | NULL    |                |
| prix_ht_p     | FLOAT        | YES  |     | NULL    |                |
| quantite_p    | INT(6)       | YES  |     | NULL    |                |
| marque_p      | VARCHAR(100) | YES  |     | NULL    |                |
| dimension_p   | VARCHAR(100) | YES  |     | NULL    |                |
| poid_p        | VARCHAR(100) | YES  |     | NULL    |                |
| img_p         | VARCHAR(100) | YES  |     | NULL    |                |
| designation_p | text         | YES  |     | NULL    |                |
| id_sc         | INT(11)      | YES  | MUL | NULL    |                |
| id_promo      | INT(10)      | YES  | MUL | NULL    |                |
| prix_ttc_p    | FLOAT        | YES  |     | NULL    |                |
+---------------+--------------+------+-----+---------+----------------+
14 ROWS IN SET (0.01 sec)
 
mysql> DESC promotion;
+----------+---------+------+-----+---------+----------------+
| FIELD    | TYPE    | NULL | KEY | DEFAULT | Extra          |
+----------+---------+------+-----+---------+----------------+
| id_promo | INT(11) | NO   | PRI | NULL    | AUTO_INCREMENT |
| taux     | FLOAT   | YES  |     | NULL    |                |
| date_deb | DATE    | YES  |     | NULL    |                |
| date_fin | DATE    | YES  |     | NULL    |                |
+----------+---------+------+-----+---------+----------------+
4 ROWS IN SET (0.01 sec) 

delimiter //
CREATE TRIGGER TTC_PROMO_UPDATE
	BEFORE UPDATE ON produit 
		FOR EACH ROW
			BEGIN
			IF new.id_promo <> old.id_promo
					
						SET new.prix_ttc_p = ((old.prix_ht_p*(100 - (SELECT taux FROM promotion,produit WHERE promotion.id_promo = produit.id_promo AND produit.ref_p = new.ref_p))/100)*(1+(produit.tva_p/100)) )
							WHERE promotion.id_promo = produit.id_promo AND produit.ref_p = new.ref_p; 
			END IF;
			END;
			//
delimiter ;