On Tue, Aug 16, 2022 at 5:02 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
> > I had that thought too, but I don't *think* it's the case. This
> > function acquires a lock on the oldest bucket page, then on the new
> > bucket page. We could deadlock if someone who holds a pin on the new
> > bucket page tries to take a content lock on the old bucket page. But
> > who would do that? The new bucket page isn't yet linked from the
> > metapage at this point, so no scan should do that. There can be no
> > concurrent writers during replay. I think that if someone else has the
> > new page pinned they probably should not be taking content locks on
> > other buffers at the same time.
>
> Agreed, the core code shouldn't do that, but somebody doing random stuff
> with pageinspect functions could probably make a query do this.
> See [1]; unless we're going to reject that bug with "don't do that",
> I'm not too comfortable with this line of reasoning.
I don't see the connection. The problem there has to do with bypassing
shared buffers, but this operation isn't bypassing shared buffers.
What sort of random things would someone do with pageinspect functions
that would hold buffer pins on one buffer while locking another one?
The functions in hashfuncs.c don't even seem like they would access
multiple buffers in total, let alone at overlapping times. And I don't
think that a query pageinspect could realistically be suspended while
holding a buffer pin either. If you wrapped it in a cursor it'd be
suspended before or after accessing any given buffer, not right in the
middle of that operation.
--
Robert Haas
EDB: http://www.enterprisedb.com