MYSQL LOCK IN SHARE MODE&FOR UPDATE

SELECT ... LOCK IN SHARE MODE sets a shared mode lock on the rows read. A shared mode lock enables other sessions to read the rows but not to modify them. The rows read are the latest available, so if they belong to another transaction that has not yet committed, the read blocks until that transaction ends. 

SELECT ... FOR UPDATE sets an exclusive lock on the rows read. An exclusive lock prevents other sessions from accessing the rows for reading or writing.