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?
