Deadlock with stored procedure generating UID (4 replies)

I have a stored procedure generating UID's from a "ticket" table, but under load I'm getting lots of deadlocks. I'm calling this procedure many times from multiple concurrent connections whenever my task needs a new UID.

BEGIN
DECLARE a_uid BIGINT(20) UNSIGNED;
START TRANSACTION;
SELECT uid INTO a_uid FROM uid_data FOR UPDATE; # Lock
INSERT INTO uid_data (stub) VALUES ('a') ON DUPLICATE KEY UPDATE uid=uid+1;
SELECT a_uid+1 AS `uid`;
COMMIT;
END

I did consider using:

BEGIN
REPLACE INTO uid_data (stub) VALUES ('a');
SELECT LAST_INSERT_ID();
END

However I wasn't sure if that would be safe with concurrent connections as there's no locking, unlike the first procedure with the `SELECT FOR UPDATE`.

Here's the table:

mysql> DESCRIBE uid_data;
+-------+---------------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-------+---------------------+------+-----+---------+----------------+
| uid | bigint(20) unsigned | NO | PRI | NULL | auto_increment |
| stub | char(1) | NO | UNI | NULL | |
+-------+---------------------+------+-----+---------+----------------+

I've setup for read-committed transaction isolation:

mysql> SHOW VARIABLES LIKE 'tx_isolation';
+---------------+-----------------+
| Variable_name | Value |
+---------------+-----------------+
| tx_isolation | READ-COMMITTED |
+---------------+-----------------+

Here's what I'm getting back from `SHOW ENGINE INNODB STATUS;`

...
... dozens and dozens of the following record locks...

Record lock, heap no 1046 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 1; hex 61; asc a;;
1: len 8; hex 00000000000335f2; asc 5 ;;

Record lock, heap no 1047 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 1; hex 61; asc a;;
1: len 8; hex 00000000000335f1; asc 5 ;;

*** (2) WAITING FOR THIS LOCK TO BE GRANTED:
RECORD LOCKS space id 13 page no 4 n bits 1120 index `stub` of table `my_db`.`uid_data` trx id 13AA89 lock_mode X waiting
Record lock, heap no 583 PHYSICAL RECORD: n_fields 2; compact format; info bits 32
0: len 1; hex 61; asc a;;
1: len 8; hex 00000000000334a8; asc 4 ;;

*** WE ROLL BACK TRANSACTION (1)

I'd be grateful if someone could explain what's happening and how they can be avoided.