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

On Sun, Mar 8, 2020 at 7:58 AM Masahiko Sawada <
masahiko(dot)sawada(at)2ndquadrant(dot)com> wrote:

> On Mon, 24 Feb 2020 at 19:08, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> >
> > On Thu, Feb 20, 2020 at 8:06 AM Andres Freund <andres(at)anarazel(dot)de>
> wrote:
> > >
> > > Hi,
> > >
> > > On 2020-02-19 11:12:18 +0530, Amit Kapila wrote:
> > > > I think till we know the real need for changing group locking, going
> > > > in the direction of what Tom suggested to use an array of LWLocks [1]
> > > > to address the problems in hand is a good idea.
> > >
> > > -many
> > >
> > > I think that building yet another locking subsystem is the entirely
> > > wrong idea - especially when there's imo no convincing architectural
> > > reasons to do so.
> > >
> >
> > Hmm, AFAIU, it will be done by having an array of LWLocks which we do
> > at other places as well (like BufferIO locks). I am not sure if we
> > can call it as new locking subsystem, but if we decide to continue
> > using lock.c and change group locking then I think we can do that as
> > well, see my comments below regarding that.
> >
> > >
> > > > It is not very clear to me that are we thinking to give up on Tom's
> > > > idea [1] and change group locking even though it is not clear or at
> > > > least nobody has proposed an idea/patch which requires that? Or are
> > > > we thinking that we can do what Tom suggested for relation extension
> > > > lock and also plan to change group locking for future parallel
> > > > operations that might require it?
> > >
> > > What I'm advocating is that extension locks should continue to go
> > > through lock.c. And yes, that requires some changes to group locking,
> > > but I still don't see why they'd be complicated.
> > >
> >
> > Fair position, as per initial analysis, I think if we do below three
> > things, it should work out without changing to a new way of locking
> > for relation extension or page type locks.
> > a. As per the discussion above, ensure in code we will never try to
> > acquire another heavy-weight lock after acquiring relation extension
> > or page type locks (probably by having Asserts in code or maybe some
> > other way).
>
> The current patch
> (v02_0001-Added-assert-to-verify-that-we-never-try-to-take-any.patch)
> doesn't check that acquiring a heavy-weight lock after page type lock,
> is that right?

No, it should do that.

> There is the path doing that: ginInsertCleanup() holds
> a page lock and insert the pending list items, which might hold a
> relation extension lock.
>

Right, I could also see that, but do you see any problem with that? I
agree that Assert should cover this case, but I don't see any fundamental
problem with that.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com