Re: SimpleLruTruncate() mutual exclusion

On Thu, Aug 1, 2019 at 6:51 PM Noah Misch  wrote:
> vac_truncate_clog() instance 1 starts, considers segment ABCD eligible to unlink
> vac_truncate_clog() instance 2 starts, considers segment ABCD eligible to unlink
> vac_truncate_clog() instance 1 unlinks segment ABCD
> vac_truncate_clog() instance 1 calls SetTransactionIdLimit()
> vac_truncate_clog() instance 1 finishes
> some backend calls SimpleLruZeroPage(), creating segment ABCD
> vac_truncate_clog() instance 2 unlinks segment ABCD
>
> Serializing vac_truncate_clog() fixes that.

I've wondered before (in a -bugs thread[1] about unexplained pg_serial
wraparound warnings) if we could map 64 bit xids to wide SLRU file
names that never wrap around and make this class of problem go away.
Unfortunately multixacts would need 64 bit support too...

[1] https://www.postgresql.org/message-id/flat/CAEBTBzuS-01t12GGVD6qCezce8EFD8aZ1V%2Bo_3BZ%3DbuVLQBtRg%40mail.gmail.com