If both users will run the select statement at the exact same time, then you’ll have a deadlock. When you work with serializable isolation level, the shared locks that were acquired by both processes will be held until the end of the transactions. Since both processes will next try to update the table, each one of them will have to wait for the other process to release the shared lock. This will cause a deadlock and one of the transactions will be rolled back.

Adi