Re: Connections hang indefinitely while taking a gin index's LWLock buffer_content lock

On Thu, Dec 6, 2018 at 12:51 PM Alexander Korotkov <aekorotkov(at)gmail(dot)com> wrote:
> So, algorithm introduced by 218f51584d5 is broken. It tries to
> guarantee that there are no inserters in the subtree by placing
> cleanup lock to subtree root, assuming that inserter holds pins on the
> path from root to leaf. But due to concurrent splits of internal
> pages the pins held can be not relevant to actual path. I don't see
> the way to fix this easily. So, I think we should revert it from back
> branches and try to reimplement that in master.
>
> However, I'd like to note that 218f51584d5 introduces two changes:
> 1) Cleanup locking only if there pages to delete
> 2) Cleanup locking only subtree root
> The 2nd one is broken. But the 1st one seems still good for me and
> useful, because in vast majority of cases vacuum doesn't delete any
> index pages. So, I propose to revert 218f51584d5, but leave there
> logic, which locks root for cleanup only once there are pages to
> delete. Any thoughts?

Can you post a patch that just removes the 2nd part, leaving the
still-correct first part?

Your proposal sounds reasonable, but I'd like to see what you have in
mind in detail before commenting.

Thanks
--
Peter Geoghegan