Re: Move PinBuffer and UnpinBuffer to atomics

On Thu, Apr 7, 2016 at 4:41 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:

> On 2016-03-31 20:21:02 +0300, Alexander Korotkov wrote:
> > ! BEGIN_BUFSTATE_CAS_LOOP(bufHdr);
> >
> > ! Assert(BUF_STATE_GET_REFCOUNT(state) > 0);
> > ! wasDirty = (state & BM_DIRTY) ? true : false;
> > ! state |= BM_DIRTY | BM_JUST_DIRTIED;
> > ! if (state == oldstate)
> > ! break;
>
> I'm doubtful that this early exit is entirely safe. None of the
> preceding operations imply a memory barrier. The buffer could previously
> have been marked dirty, but cleaned since. It's pretty critical that we
> re-set the dirty bit (there's no danger of loosing it with a barrier,
> because we hold an exclusive content lock).
>

Oh, I get it.

> Practically the risk seems fairly low, because acquiring the exclusive
> content lock will have implied a barrier. But it seems unlikely to have
> a measurable performance effect to me, so I'd rather not add the early
> exit.
>

Ok, let's just remove it.

------
Alexander Korotkov
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The Russian Postgres Company