Re: Atomics for heap_parallelscan_nextpage()

From: Andres Freund <andres(at)anarazel(dot)de>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Amit Khandekar <amitdkhan(dot)pg(at)gmail(dot)com>, Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>
Subject: Re: Atomics for heap_parallelscan_nextpage()
Date: 2017-08-16 17:10:25
Message-ID: 20170816171025.vfwywwgunldgc5fk@alap3.anarazel.de
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Robert, Tom,

On 2017-08-16 09:55:15 -0700, Andres Freund wrote:
> > Not sure if this is your bug or if it's exposing a pre-existing
> > deficiency in the atomics code, viz, failure to ensure that
> > pg_atomic_uint64 is actually a 64-bit-aligned type. Andres?

> I suspect it's the former. Suspect that the shared memory that holds
> the "parallel desc" isn't properly aligned:

Or, well, a mixture of both, because it seems like a deficiency in the
shm_toc, code, rather than the atomics code.

> Afaict shm_create/shm_toc_allocate don't actually guarantee that the end
> of the toc's memory is suitably aligned. But I didn't yet have any
> coffee, so ...

Robert, I'm not quite sure what the intended behaviour of shm_toc is wrt
alignment. I see that individual chunks are BUFFERALIGNed (both during
estimation, and allocation). But I don't see how the size of the entire
toc is aligned, which seems a requirement, given we allocate from the
end.
Seems like we'd just have to add alignment of the total size to
shm_toc_estimate()?

Greetings,

Andres Freund

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