SELECT 上的 MySQL InnoDB 死锁与排他锁(FOR UPDATE)

I do this to ensure only once instance of this process is running (pseudo code php/mysql innodb):

START TRANSACTION
$rpid = SELECT `value` FROM locks WHERE name = "lock_name" FOR UPDATE
$pid = posix_getpid();
if($rpid > 0){
  $isRunning = posix_kill($rpid, 0);
  if(!$isRunning){ // isRunning
    INSERT INTO locks values('lock_name', $pid) ON DUPLICATE KEY UPDATE `value` = VALUES(`value`)
  }else{
    ROLLBACK
    echo "Allready running...
";
    exit();
  }
}else{ // if rpid == 0 -
  INSERT INTO locks values('lock_name', $pid) ON DUPLICATE KEY UPDATE `value` = VALUES(`value`)
}
COMMIT

...............

//free the pid
INSERT INTO locks values('lock_name', 0) ON DUPLICATE KEY UPDATE `value` = VALUES(`value`)

表锁包含以下字段:

id - primary, autoinc
name - varchar(64) unique key
description - text
value - text

我相信从 START TRANSACTIN 到 COMMIT/ROLLBACK 的时间真的是几毫秒 - 甚至没有足够的时间来超时.这段代码怎么可能会出现死锁?我不在此事务中使用其他表.看起来死锁是不可能的.如果 2 个进程同时启动，第一个获得该行锁的进程将继续进行，另一个将等待锁被释放.如果在 1 分钟内没有释放锁，则错误是超时"，而不是死锁.

$myPid = posix_getpid();
$gotIt = false;
while(true){
  START TRANSACTION;
  $pid = SELECT ... FOR UPDATE; // read pid and get lock on it
  if(mysql_num_rows($result) == 0){
    ROLLBACK;// release lock to avoid deadlock
    INSERT IGNORE INTO locks VALUES('lockname', $myPid);
  }else{
    //pid existed, no insert is needed
    break;
  }
}

if($pid != $myPid){ //we did not insert that
  if($pid>0 && isRunning($pid)){
    ROLLBACK;
    echo 'another process is running';
    exit;
  }{
    // no other process is running - write $myPid in db
    UPDATE locks SET value = $myPid WHERE name = 'lockname'; // update is safe
    COMMIT;
  }
}else{
  ROLLBACK; // release lock
}