Greg Stark <gsstark@mit.edu> writes:
> On Tue, Feb 22, 2011 at 12:55 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>> A little OT, but ISTM that the buffer pin mechanism by its nature is
>> prone to lock upgrade hazards.
> Except that pins don't block exclusive locks so there's no deadlock risk.
> The oddity here is on Vacuums super-exclusive "lock" which is the real
> equivalent of an "exclusive lock". However there's the added bonus
> that there can only be one vacuum on a table at a time. That makes it
> safe
We have seen deadlocks arising from this type of scenario:
autovac has vacuum lock on table Xautovac blocks waiting for cleanup lock on buffer B in Xprocess P has pin on B due to
asuspended query (eg cursor)P tries to get exclusive lock on X, is blocked by autovac's lock
The heavyweight-lock manager fails to recognize deadlock because it
doesn't know about the buffer-level LWLock.
> It might be interesting to have autovacuum skip a block it finds
> pinned for too long.
+1, although as somebody pointed out nearby, this will only be legal if
it's not a vacuum-to-prevent-wraparound situation.
> Incidentally, even if we allowed multiple vacuum processes per table I
> think it could be coded to be safe as long as each vacuum only needs
> to acquire the super exclusive lock on a single block at a time and
> doesn't try to acquire other locks while holding it.
IIRC, it's cleaning the indexes that is problematic.
regards, tom lane