Re: SSI work for 9.1

On Wed, Jun 08, 2011 at 09:17:04PM -0500, Kevin Grittner wrote:
> A patch is attached which just covers the predicate lock acquisition,
> where a snapshot is available without too much pain.  There are two
> functions which acquire predicate locks where a snapshot was not
> readily available: _bt_search() and _bt_get_endpoint().  Not only was
> it not clear how to get a snapshot in, it was not entirely clear from
> reading the code that we need to acquire predicate locks here.  Now,
> I suspect that we probably do, because I spent many long hours
> stepping through gdb to pick the spots where they are, but that was
> about a year ago and my memory of the details has faded.

For _bt_search(), the lock calls should move to _bt_first() where the
ScanDesc is available. This also keeps us from trying to take locks
during _bt_pagedel(), which is only called during vacuum and recovery.

The call in _bt_get_endpoint() seems unnecessary, because after it
returns, _bt_endpoint() takes the same lock. The only other callers of
_bt_get_endpoint() are _bt_pagedel() and _bt_insert_parent(), neither
of which should take predicate locks.

I've updated the patch, attached.

Dan

-- 
Dan R. K. Ports              MIT CSAIL                http://drkp.net/