Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager

On Sat, Mar 7, 2020 at 8:53 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Dilip Kumar <dilipbalaut(at)gmail(dot)com> writes:
> > I think instead of the flag we need to keep the counter because we can
> > acquire the same relation extension lock multiple times.
>
> Uh ... what? How would that not be broken usage on its face?

Basically, if we can ensure that while holding the relation extension
lock we will not wait for any other lock then we can ignore in the
deadlock detection path so that we don't detect the false deadlock due
to the group locking mechanism. So if we are already holding the
relation extension lock and trying to acquire the same lock-in same
mode then it can never wait so this is safe.

> I continue to think that we'd be better off getting all of this
> out of the heavyweight lock manager. There is no reason why we
> should need deadlock detection, or multiple holds of the same
> lock, or pretty much anything that LWLocks don't give you.

Right, we never need deadlock detection for this lock. But, I think
there are quite a few cases where we have multiple holds at the same
time. e.g, during RelationAddExtraBlocks, while holding the relation
extension lock we try to update the block in FSM and FSM might need to
add extra FSM block which will again try to acquire the same lock.

But, I think the main reason for not converting it to an LWLocks is
because Andres has a concern about inventing new lock mechanism as
discuss upthread[1]

[1] https://www.postgresql.org/message-id/20200220023612.c44ggploywxtlvmx%40alap3.anarazel.de

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com