Re: Reduce ProcArrayLock contention

On Fri, Aug 21, 2015 at 2:31 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> No, if it's paired like that, I don't think it's allowed to fail.
>
> But, as the code stands, there's absolutely no guarantee you're not
> seeing something like:
> P1: a = 0;
> P1: b = 0;
> P1: PGSemaphoreLock(&P1);
> P2: a = 1;
> P2: PGSemaphoreUnlock(&P1); -- unrelated, as e.g. earlier by ProcSendSignal
> P1: Assert(a == b == 1);
> P2: b = 1;
> P2: PGSemaphoreUnlock(&P1);
>
> if the pairing is like this there's no guarantees anymore, right? Even
> if a and be were set before P1's assert, the thing would be allowed to
> fail, because the store to a or b might each be visible since there's no
> enforced ordering.

Hmm, I see your point. So I agree with your proposed fix then. That
kinda sucks that we have to do all those gymnastics, though: that's a
lot more complicated than what we have right now.

--
Robert Haas
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