Is this MariaDB index creation locking table? What lock?

I like to add a simple no primary index to an InnoDB table on MariaDB-5.5.68 (CentOS) in production.

I found no reliable information if the following ALTER TABLE is read or write locking the table or not. As it will run for a few minutes (50GB database), I want to find out if I need to shut down the service or not.

ALTER TABLE tbllog ADD INDEX idx_LogDate(logdate);

Best would be, if no locking will happen.

I already found the LOCK=NONE option, but no information about if this is working in MariaDB 5.5.68 :-(

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