Re: [HACKERS] Moving relation extension locks out of heavyweight lock manager - Mailing list pgsql-hackers

On Wed, Mar 4, 2020 at 11:45 AM Mahendra Singh Thalor
<mahi6run@gmail.com> wrote:
>
> On Mon, 24 Feb 2020 at 15:39, Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Feb 20, 2020 at 8:06 AM Andres Freund <andres@anarazel.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.
>
> Thoughts?

+static void AssertAnyExtentionLockHeadByMe(void);

+/*
+ * AssertAnyExtentionLockHeadByMe -- test whether any EXTENSION lock held by
+ * this backend.  If any EXTENSION lock is hold by this backend, then assert
+ * will fail.  To use this function, assert should be enabled.
+ */
+void AssertAnyExtentionLockHeadByMe()
+{

Some minor observations on 0002.
1. static is missing in a function definition.
2. Function name should start in new line after function return type
in function definition, as per pg guideline.
+void AssertAnyExtentionLockHeadByMe()
->
void
AssertAnyExtentionLockHeadByMe()

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