Which modification of the Two-Phase Locking Protocol requires transactions to hold all locks until they commit or abort?

The modification of the Two-Phase Locking Protocol that requires transactions to hold all locks until they commit or abort is known as Strict Two-Phase Locking (S2PL).

Here are the steps to understand this:

1. Two-Phase Locking Protocol (2PL): This is a protocol in concurrency control of DBMS which allows transactions to acquire a lock prior to accessing data. The protocol has two phases - the expanding phase where locks are acquired and no locks are released, and the shrinking phase where acquired locks are released and no new locks are acquired.

2. Strict Two-Phase Locking (S2PL): This is a modification of the 2PL protocol. In S2PL, a transaction holds all its exclusive locks till it commits or aborts. This means, once a transaction releases a lock, it cannot request any other locks. This is done to prevent cascading rollbacks, which can occur in the standard 2PL protocol.

So, the answer to your question is the Strict Two-Phase Locking (S2PL) protocol.