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

On Wed, 4 Mar 2020 at 12:03, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Wed, Mar 4, 2020 at 11:45 AM Mahendra Singh Thalor
> <mahi6run(at)gmail(dot)com> wrote:
> >
> > On Mon, 24 Feb 2020 at 15:39, 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).
> > > b. Change lock.c so that group locking is not considered for these two
> > > lock types. For ex. in LockCheckConflicts, along with the check (if
> > > (proclock->groupLeader == MyProc && MyProc->lockGroupLeader == NULL)),
> > > we also check lock->tag and call it a conflict for these two locks.
> > > c. The deadlock detector can ignore checking these two types of locks
> > > because point (a) ensures that those won't lead to deadlock. One idea
> > > could be that FindLockCycleRecurseMember just ignores these two types
> > > of locks by checking the lock tag.
> >
> > Thanks Amit for summary.
> >
> > Based on above 3 points, here attaching 2 patches for review.
> >
> > 1. v01_0001-Conflict-EXTENTION-lock-in-group-member.patch (Patch by Dilip Kumar)
> > Basically this patch is for point b and c.
> >
> > 2. v01_0002-Added-assert-to-verify-that-we-never-try-to-take-any.patch
> > (Patch by me)
> > This patch is for point a.
> >
> > After applying both the patches, make check-world is passing.
> >
> > We are testing both the patches and will post results.

Hi all,

I am planing to test below 3 points on v1 patch set:

1. We will check that new added assert can be hit by hacking code
(while holding extension lock, try to take any heavyweight lock)
2. In FindLockCycleRecurseMember, for testing purposes, we can put
additional loop to check that for all relext holders, there must not
be any outer edge.
3. Test that group members are not granted the lock for the relation
extension lock (group members should conflict).

Please let me know your thoughts to test this patch.

--
Thanks and Regards
Mahendra Singh Thalor
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