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

On 03/04/2018 10:27 AM, Thomas Munro wrote:
> On Sun, Mar 4, 2018 at 5:40 PM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> Could shm_mq_detach_internal() need a pg_write_barrier() before it
>> writes mq_detached = true, to make sure that anyone who observes that
>> can also see the most recent increase of mq_bytes_written?
>
> I can reproduce both failure modes (missing tuples and "lost contact")
> in the regression database with the attached Python script on my Mac.
> It takes a few minutes and seems to be happen sooner when my machine
> is also doing other stuff (playing debugging music...).
>
> I can reproduce it at 34db06ef9a1d7f36391c64293bf1e0ce44a33915
> "shm_mq: Reduce spinlock usage." but (at least so far) not at the
> preceding commit.
>
> I can fix it with the following patch, which writes XXX out to the log
> where it would otherwise miss a final message sent just before
> detaching with sufficiently bad timing/memory ordering. This patch
> isn't my proposed fix, it's just a demonstration of what's busted.
> There could be a better way to structure things than this.
>

I can confirm this resolves the issue for me. Before the patch, I've
seen 112 failures in ~11500 runs. With the patch I saw 0 failures, but
about 100 messages XXX in the log.

So my conclusion is that your analysis is likely correct.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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