Can I use row-level locks to sequence READ COMMITTED transactions?

If I use a READ COMMITTED transaction, the documentation says that a
query in my transaction may see changes which were committed by other
transactions after my transaction started.

My question is, is it guaranteed that a SELECT in my transaction will
see changes previously committed by other transactions, or is it only a
possibility?

By 'previously committed', I mean 'committed by a transaction which held
a row-level lock which my transaction has since obtained'.



For example, if I run the following transaction many times
simultaneously,

    BEGIN;
    SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
    SELECT 1 FROM walls WHERE wall_id = 3 FOR UPDATE;

    INSERT INTO bottles (
            wall_id,
            number_seen)
        VALUES (
            3,
            (SELECT COUNT(*) FROM bottles WHERE wall = 3)
        );
    COMMIT;

can I end up with two rows in the 'bottles' table with the same value
for 'seen'? Or do I have a guarantee that after one instance of the
transaction has released the 'SELECT FOR UPDATE' lock, any other
instance will see the data that it has committed?


-M-