Re: ALTER TABLE lock strength reduction patch is unsafe

* Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> Stephen Frost <sfrost(at)snowman(dot)net> writes:
> > I don't have too much of an issue with the above, but I would like to
> > have us figure out a solution to the deadlock problem with parallel
> > pg_dump. The issue arises when pg_dump gets an AccessShareLock and then
> > another process attempts to acquire an AccessExclusiveLock, which then
> > blocks, and then the pg_dump worker process tries to get its
> > AccessShareLock- we end up not being able to make any progress on
> > anything at that point.
>
> The original ASL was acquired by the pg_dump master, right?

Yup. It goes through and gets ASLs on everything first.

> > One suggestion that was discussed at PGConf.EU was having processes
> > which share the same snapshot (the pg_dump master and worker processes)
> > able to either share the same locks or at least be able to "jump" the
> > lock queue (that is, the worker process wouldn't have to wait being the
> > AEL to get an ASL, since the ASL was already aquired for the snapshot
> > which was exported and shared with it).
>
> Yeah, it seems like we need lock export not only snapshot export to make
> this work nicely. But that sounds orthogonal to the issues being
> discussed in this thread.

Indeed, just figured I'd mention it since we're talking about
pg_dump-related locking.

Thanks,

Stephen