Re: Move PinBuffer and UnpinBuffer to atomics

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).

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.