Re: [HACKERS] ginInsertCleanup called from vacuum could still miss tuples to be deleted

On Thu, Nov 16, 2017 at 12:44 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> I would like to get rid of that LockPage() call, for sure, because
> it's problematic in terms of allowing writes in parallel mode.
> However, I think the reason here is the same as why the hash AM used
> to use them. If you use a buffer lock, you really can't -- or
> shoudn't, at least -- hold it across a whole series of operations,
> because anyone waiting for that lock is waiting *uninterruptibly*.

I'm well aware of that.

> The hash AM wanted to iterate through all of the pages in a bucket
> chain and do something to each one while preventing concurrent scans;
> the need here is similar. Aside from the uninterruptible-wait
> problem, such coding patterns are extremely prone to deadlock. If
> there's any chance that a process waiting for the buffer lock you hold
> might be holding a buffer lock you try to acquire, you have got a
> problem.

But there is only ever one page locked, the meta-page. And, it's
always an ExclusiveLock. I don't see much use for deadlock avoidance.

In any case, it's unusual to have a patch that uses LockPage() without
explaining the choice.

--
Peter Geoghegan