LOCK IN SHARE MODE is specified in a SELECT statement, MariaDB will wait until all transactions that have modified the rows are committed. Then, a write lock is acquired. All transactions can read the rows, but if they want to modify them, they have to wait until your transaction is committed.
InnoDB/XtraDB supports row-level locking. selected rows can be locked using
LOCK IN SHARE MODE or
FOR UPDATE. In both cases, a lock is acquired on the rows read by the query, and it will be released when the current transaction is committed.
autocommit is set to 1, the LOCK IN SHARE MODE and FOR UPDATE clauses have no effect.