> what happens if … reader here stores the old unloggedLSN value > to control file and the server restarts (clean exit). So, the old >value is loaded back to unloggedLSN upon restart and the callers of > GetFakeLSNForUnloggedRel() will see an old/repeated value too. Is it a > problem?
First, a clarification. The value being saved is the “next” unlogged LSN, not one which has already been used. (we are doing “fetch and add”, not “add and fetch”)
You have a good point about shutdown and startup. It is vital we don’t repeat an unlogged LSN. This situation could easily happen If other readers were active while we were shutting down.
>With an atomic variable, it is guaranteed that the readers >don't see a torn-value, but no synchronization is provided.
The atomic increment also ensures the sequence of values is correct, specifically we don’t see repeated values like we might with a conventional increment. As a side effect, the instruction enforces a memory barrier, but we are not relying on a barrier in this case.
