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. -- Michael
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