How to do compare-and-increment different locations with in memory MVCC?

Question

I'm reading the paper An Empirical Evaluation of In-Memory Multi-Version Concurrency Control which is about implementations of MVCC for in-memory databases. Section 3.3 on Two-phase Locking (MV2PL) says:

To perform a read operation on a tuple A, the DBMS searches for a visible version by comparing a transaction’s T-id with the tuples’ begin-ts field. If it finds a valid version, then the DBMS increments that tuple’s read-cnt field if its txn-id field is equal to zero (meaning that no other transaction holds the write lock)

Here is the image in Figure 2c:

The confusion point is about how it actually does this operation atomically: Read location A (txn-id), if its value is equal to 0 then increment location B (read-cnt)

Atomic operation in C/C++ does compare-and-swap only for a single memory location, however, in this case we are dealing with two memory locations. Is there any other CPU intrinsic for this operation, or another trick is employed?

Answer 1

I asked the question directly from paper authors, and they replied to me:

txn-id and read-cnt are 4-bytes each (not 8 bytes as I thought before), thus it is possible to compare-and-swap in one step.