Re: hash_xlog_split_allocate_page: failed to acquire cleanup lock

On Fri, Sep 30, 2022 at 12:05 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> My problem with this approach is that the whole cleanup lock is hugely
> misleading as-is.

While nbtree VACUUM does use cleanup locks, they don't protect the
index structure itself -- it actually functions as an interlock
against concurrent TID recycling, which might otherwise confuse
in-flight index scans. That's why we need cleanup locks for VACUUM,
but not for index deletions, even though the physical modifications
that are performed to physical leaf pages are identical (the WAL
records are almost identical). Clearly the use of cleanup locks is not
really about protecting the leaf page itself -- it's about using the
physical leaf page as a proxy for the heap TIDs contained therein. A
very narrow protocol with a very specific purpose.

More generally, cleanup locks exist to protect transient references
that point into a heap page. References held by one backend only. A
TID, or a HeapTuple C pointer, or something similar. Cleanup locks are
not intended to protect a physical data structure in the heap, either
-- just a reference/pointer that points to the structure. There are
implications for the physical page structure itself, of course, but
that seems secondary. The guarantees are often limited to "never allow
the backend holding the pin to become utterly confused".

I am skeptical of the idea of using cleanup locks for anything more
ambitious than this. Especially in index AM code. It seems
uncomfortably close to "a buffer lock, but somehow also not a buffer
lock".

--
Peter Geoghegan