Re: select_parallel test failure: gather sometimes losing tuples (maybe during rescans)?

On Sun, Mar 4, 2018 at 4:46 PM, Thomas Munro
<thomas(dot)munro(at)enterprisedb(dot)com> wrote:
> Thanks! Here are a couple of patches. I'm not sure which I prefer.
> The "pessimistic" one looks simpler and is probably the way to go, but
> the "optimistic" one avoids doing an extra read until it has actually
> run out of data and seen mq_detached == true.
>
> I realised that the pg_write_barrier() added to
> shm_mq_detach_internal() from the earlier demonstration/hack patch was
> not needed... I had a notion that SpinLockAcquire() might not include
> a strong enough barrier (unlike SpinLockRelease()), but after reading
> s_lock.h I think it's not needed (since you get either TAS() or a
> syscall-based slow path, both expected to include a full fence). I
> haven't personally tested this on a weak memory order system.

The optimistic approach seems a little bit less likely to slow this
down on systems where barriers are expensive, so I committed that one.
Thanks for debugging this; I hope this fixes it, but I guess we'll
see.

--
Robert Haas
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