Re: missing LockBuffer(buffer, BUFFER_LOCK_SHARE) in trigger.c GetTupleForTrigger?

On Fri, Nov 30, 2012 at 5:28 PM, Andres Freund <andres(at)2ndquadrant(dot)com>wrote:

>
> > >
> > That seems to be safe to me. Anything thats been read above can't really
> > change. The tuple is already locked, so a concurrent update/delete has to
> > wait on us. We have a pin on the buffer, so VACUUM or HOT-prune can't
> > happen either. I can't see any other operation that can really change
> those
> > fields.
>
> We only get the pin right there, I don't see any preexisting pin. Which
> means we might have just opened a page thats in the process of being
> pruned/vacuumed by another backend.
>

Hmm. Yeah, you're right. That is a possible risky scenario. Even though
cleanup lock waits for all pins to go away, it will work only if every
reader takes at least a SHARE lock unless it was continuously holding a pin
on a buffer (in which case its OK to drop lock and read a tuple body
without reacquiring it again). Otherwise, as you rightly pointed out, we
could actually be reading a page which being actively cleaned up and tuples
are being moved around.

> I think a concurrent heap_(insert|update)/PageAddItem might actually be
> already dangerous because it might move line pointers around
>
>
I don't we move the line pointers around ever because the indexes will be
pointing to them. But the vacuum/prune is dangerous enough to require a
SHARE lock here in any case.

Thanks,
Pavan

--
Pavan Deolasee
http://www.linkedin.com/in/pavandeolasee