Re: Conflict detection for update_deleted in logical replication

On Tue, Jul 1, 2025 at 10:53 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> On Tue, Jul 1, 2025 at 10:31 AM Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
> >
> > On Mon, Jun 30, 2025 at 6:59 PM Zhijie Hou (Fujitsu)
> > <houzj(dot)fnst(at)fujitsu(dot)com> wrote:
> > >
> > > On Mon, Jun 30, 2025 at 7:22 PM Amit Kapila wrote:
> > > >
> >
> > I was looking at 0001, it mostly looks fine to me except this one
> > case. So here we need to ensure that commits must be acquired after
> > marking the flag, don't you think we need to ensure strict statement
> > ordering using memory barrier, or we think it's not required and if so
> > why?
> >

Good point. I also think we need a barrier here, but a write barrier
should be sufficient as we want ordering of two store operations.

> > RecordTransactionCommitPrepared()
> > {
> > ..
> > + MyProc->delayChkptFlags |= DELAY_CHKPT_IN_COMMIT;
> > +
> > + /*
> > + * Note it is important to set committs value after marking ourselves as
> > + * in the commit critical section (DELAY_CHKPT_IN_COMMIT). This is because
> > + * we want to ensure all transactions that have acquired commit timestamp
> > + * are finished before we allow the logical replication client to advance
> > + * its xid which is used to hold back dead rows for conflict detection.
> > + * See maybe_advance_nonremovable_xid.
> > + */
> > + committs = GetCurrentTimestamp();
> > }
>
> I'm unsure whether the function call inherently acts as a memory
> barrier, preventing the compiler from reordering these operations.
> This needs to be confirmed.
>

As per my understanding, function calls won't be a memory barrier. In
this regard, we need a similar change in RecordTransactionCommit as
well.

--
With Regards,
Amit Kapila.