Re: ALTER TABLE lock strength reduction patch is unsafe

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?

> 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.

regards, tom lane