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

On Mon, Nov 13, 2017 at 8:08 PM, Peter Geoghegan <pg@bowt.ie> wrote:

> On Mon, Nov 13, 2017 at 6:56 PM, Masahiko Sawada <sawada.mshk@gmail.com>
> wrote:
> >    /*
> >     * We would like to prevent concurrent cleanup process. For that we
> will
> >     * lock metapage in exclusive mode using LockPage() call. Nobody other
> >     * will use that lock for metapage, so we keep possibility of
> concurrent
> >     * insertion into pending list
> >     */
> >
> > So I conjecture that this has been introduced for not the reason why
> > we need to detect deadlock but the reason why we need to a different
> > lock from the lock used by insertion into pending list.
>
> I understood that much, but I think that we need to detect problems
> and recover from them (something like _bt_page_recyclable()), rather
> than preventing them with pessimistic locking -- or, at least, there
> is no reason I know to think that the HW lock is sufficient, and I am
> tempted to go that way to fix this. Commit e9568083, which added the
> feature that led to commit e2c79e14, may itself be the basic problem
> here.


e2c79e14 was to fix a pre-existing bug, but probably e9568083 made that bug
easier to hit than it was before.  (Which is not to say that  e9568083
can't contain bugs of its own, of course)


> * According to the GIN README, the pending list cleanup by VACUUM has
> a super-exclusive lock on the root, to block out concurrent inserters
> (that hold a pin on the root throughout). That's why it was/is okay
> that VACUUM recycled pending list pages without a RecentGlobalXmin
> interlock. Not so easy, but still not hard.
>

The only reference to super-exclusive lock in
src/backend/access/gin/README, that I can find, is about posting trees, not
pending lists.  Can you quote or give line numbers of the section you are
referring to?

Cheers,

Jeff