Hi, On 2019-05-07 12:25:43 +0900, Michael Paquier wrote: > On Tue, May 07, 2019 at 12:07:56PM +0900, Michael Paquier wrote: > > Now, what do we do about the potential deadlock issues in > > WaitForOlderSnapshots? The attached is an isolation test able to > > reproduce the deadlock within WaitForOlderSnapshots() with two > > parallel REINDEX CONCURRENTLY. I'd like to think that the best way to > > do that would be to track in vacuumFlags the backends running a > > REINDEX and just exclude them from GetCurrentVirtualXIDs() because > > we don't actually care about missing index entries in this case like > > VACUUM. But it looks also to me that is issue is broader and goes > > down to utility commands which can take a lock on a table which cannot > > be run in transaction blocks, hence code paths used by CREATE INDEX > > CONCURRENTLY and DROP INDEX CONCURRENTLY could also cause a similar > > deadlock, no? > > More to the point, one can just do that without REINDEX: > - session 1: > create table aa (a int); > begin; > lock aa in row exclusive mode; > - session 2: > create index concurrently aai on aa(a); --blocks > - session 3: > create index concurrently aai2 on aa(a); --blocks > - session 1: > commit; > > Then session 2 deadlocks while session 3 finishes correctly. I don't > know if this is a class of problems we'd want to address for v12, but > if we do then CIC (and DROP INDEX CONCURRENTLY?) could benefit from > it.
This seems like a pre-existing issue to me. We probably should improve that, but I don't think it has to be tied to 12. Greetings, Andres Freund
