| From: | Alexander Korotkov <aekorotkov(at)gmail(dot)com> |
|---|---|
| To: | Andres Freund <andres(at)anarazel(dot)de> |
| Cc: | Xuneng Zhou <xunengzhou(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, Peter Eisentraut <peter(at)eisentraut(dot)org>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, Álvaro Herrera <alvherre(at)kurilemu(dot)de>, Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, Michael Paquier <michael(at)paquier(dot)xyz>, jian he <jian(dot)universality(at)gmail(dot)com>, Tomas Vondra <tomas(at)vondra(dot)me>, Yura Sokolov <y(dot)sokolov(at)postgrespro(dot)ru> |
| Subject: | Re: Implement waiting for wal lsn replay: reloaded |
| Date: | 2026-04-08 07:31:07 |
| Message-ID: | CAPpHfdv4C_G9==YT6ic9wftwTmPzcfh8PJ5NPti3_OSzP=_RaQ@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Wed, Apr 8, 2026 at 3:50 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2026-04-08 02:30:44 +0300, Alexander Korotkov wrote:
> > On Tue, Apr 7, 2026 at 6:55 PM Xuneng Zhou <xunengzhou(at)gmail(dot)com> wrote:
> > I agree to change in WaitLSNWakeup(), memory barrier looks necessary there.
> > Regarding GetCurrentLSNForWaitType(), I don't think barrier is needed
> > here, nor think it makes things clearer. I think it would be enough
> > to comment that LWLock operations in addLSNWaiter()/deleteLSNWaiter()
> > provide necessary barriers.
>
> That's sufficient for the first iteration, but what guarantees it once you do
> WaitLatch()? That's likely going to imply a barrier somewhere in the kernel,
> but I don't think there's any actual guarantee.
After WaitLatch(), ResetLatch() contains memory barrier. And as I
understand, this memory barrier includes guarantees for reading fresh
values after WaitLatch() in typical latch usage scenario. However, I
see in WaitForLSN() we can exit from WaitLatch() on timeout, and then
potentially exit from loop on timeout without rechecking for the most
fresh LSN. I suppose we can just do ResetLatch() unconditionally to
fix that.
------
Regards,
Alexander Korotkov
Supabase
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