Re: Get rid of WALBufMappingLock

On Sun, Mar 2, 2025 at 1:58 PM Alexander Korotkov <aekorotkov@gmail.com> wrote:
>
> On Fri, Feb 28, 2025 at 3:13 PM Álvaro Herrera <alvherre@alvh.no-ip.org> wrote:
> > On 2025-Feb-28, Michael Paquier wrote:
> >
> > > Saying that, I have also done similar tests with your v12 for a couple
> > > of hours and this looks stable under installcheck-world.  I can see
> > > that you've reworked quite a bit the surroundings of InitializedFrom
> > > in this one.  If you apply that once again at some point, the
> > > buildfarm will be judge in the long-term, but I am rather confident by
> > > saying that the situation looks better here, at least.
> >
> > Heh, no amount of testing can prove lack of bugs; but for sure "it looks
> > different now, so it must be correct" must be the weakest proof of
> > correctness I've heard of!
>
> Michael just volunteered to help Yura and me with testing.  He wan't
> intended to be reviewer.  And he reported that tests looks much more
> stable now.  I think he is absolutely correct with this.

Nevertheless, I don't think the bug has gone in v12.  I managed to
reproduce it on my local Raspberry PI 4.  The attached version of
patch fixes the bug for me.  It adds memory barriers surrounding
pg_atomic_compare_exchange_u64().  That certainly not right given this
function should already provide full memory barrier semantics.  But my
investigation shows it doesn't.  I'm going to start a separate thread
about this.

Also, new version of patch contains fix of potential integer overflow
during OldPageRqstPtr computation sent off-list my me by Yura.

------
Regards,
Alexander Korotkov
Supabase