I pushed the "copy" pointer now, except that I renamed it to "insert", which is what we call the operation being tracked. I also added some comments.
One perhaps significant change from what Bharath submitted is that we now use the return value from monotonic advance to return an updated value of finishedUpto from WaitXLogInsertionsToFinish. I think this is a useful optimization: the only caller of WaitXLogInsertionsToFinish which cares about its return value is XLogFlush, which AFAICS benefits from having a more up-to-date value. Also, I did adopt your (Jeff's) idea of using a barrier when reading logInsertResult in WaitXLogInsertionsToFinish. If I understand correctly, this is actually useful: if some other CPU core was also recently busy waiting for xlog insertions and wrote an updated version of logInsertResult that's past what we last saw, we benefit from an updated value because we can skip the spinlock dance and WAL-insert-var stuff altogether. With no barrier, we would potentially see an outdated value of logInsertResult and waste a lot of effort to get it updated. -- Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/ "Those who use electric razors are infidels destined to burn in hell while we drink from rivers of beer, download free vids and mingle with naked well shaved babes." (http://slashdot.org/comments.pl?sid=44793&cid=4647152)
