MySQL InnoDB dead lock on SELECT with exclusive lock (FOR UPDATE)

Question

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 n         


        

Answer 1

Just figured it out thanks to Quassnoi's answer...

I can do:

$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
}