Re: hash_xlog_split_allocate_page: failed to acquire cleanup lock

On Fri, Oct 14, 2022 at 2:25 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> >
> > Attached are two patches. The first patch is what Robert has proposed
> > with some changes in comments to emphasize the fact that cleanup lock
> > on the new bucket is just to be consistent with the old bucket page
> > locking as we are initializing it just before checking for cleanup
> > lock. In the second patch, I removed the acquisition of cleanup lock
> > on the new bucket page and changed the comments/README accordingly.
> >
> > I think we can backpatch the first patch and the second patch can be
> > just a HEAD-only patch. Does that sound reasonable to you?
>
> Not particularly, no. I don't understand how "overwrite a page and then get a
> cleanup lock" can sensibly be described by this comment:
>
> > +++ b/src/backend/access/hash/hashpage.c
> > @@ -807,7 +807,8 @@ restart_expand:
> > * before changing the metapage's mapping info, in case we can't get the
> > * disk space. Ideally, we don't need to check for cleanup lock on new
> > * bucket as no other backend could find this bucket unless meta page is
> > - * updated. However, it is good to be consistent with old bucket locking.
> > + * updated and we initialize the page just before it. However, it is just
> > + * to be consistent with old bucket locking.
> > */
> > buf_nblkno = _hash_getnewbuf(rel, start_nblkno, MAIN_FORKNUM);
> > if (!IsBufferCleanupOK(buf_nblkno))
>
> This is basically saying "I am breaking basic rules of locking just to be
> consistent", no?
>

Fair point. How about something like: "XXX Do we really need to check
for cleanup lock on the new bucket? Here, we initialize the page, so
ideally we don't need to perform any operation that requires such a
check."?

Feel free to suggest something better.

--
With Regards,
Amit Kapila.