From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
cc: Robert Haas <robertmhaas@gmail.com>, pgsql-hackers@lists.postgresql.org
Subject: Re: Atomic operations within spinlocks
In-reply-to: <20200606023103.avzrctgv7476xj7i@alap3.anarazel.de>
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 <20200604185525.wuyjtkluppj4eddr@alap3.anarazel.de>
 <1256701.1591297654@sss.pgh.pa.us>
 <20200605023302.g6v3ydozy5txifji@alap3.anarazel.de>
 <20200606001926.zin5getvvhqppnm2@alap3.anarazel.de>
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Comments: In-reply-to Andres Freund <andres@anarazel.de>
	message dated "Fri, 05 Jun 2020 19:31:03 -0700"
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Date: Fri, 05 Jun 2020 22:52:47 -0400
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Andres Freund <andres@anarazel.de> writes:
> On 2020-06-05 21:01:56 -0400, Tom Lane wrote:
>> At some point I think we'll have to give up --disable-spinlocks; it's
>> really of pretty marginal use (how often does anyone port PG to a new
>> CPU type?) and the number of weird interactions it adds in this area
>> seems like more than it's worth.

> Indeed. And any new architecture one would port PG to would have good
> enough compiler intrinsics to make that trivial. I still think it'd make
> sense to have a fallback implementation using compiler intrinsics...

> And I think we should just require 32bit atomics at the same time. Would
> probably kill gaur though.

Not only gaur.  A quick buildfarm survey finds these active members
reporting not having 32-bit atomics:

 anole         | 2020-06-05 11:20:17 | pgac_cv_gcc_atomic_int32_cas=no
 chipmunk      | 2020-05-29 22:27:56 | pgac_cv_gcc_atomic_int32_cas=no
 curculio      | 2020-06-05 22:30:06 | pgac_cv_gcc_atomic_int32_cas=no
 frogfish      | 2020-05-31 13:00:25 | pgac_cv_gcc_atomic_int32_cas=no
 gaur          | 2020-05-19 13:33:25 | pgac_cv_gcc_atomic_int32_cas=no
 gharial       | 2020-06-05 12:41:14 | pgac_cv_gcc_atomic_int32_cas=no
 hornet        | 2020-06-05 09:11:26 | pgac_cv_gcc_atomic_int32_cas=no
 hoverfly      | 2020-06-05 22:06:14 | pgac_cv_gcc_atomic_int32_cas=no
 locust        | 2020-06-05 10:14:29 | pgac_cv_gcc_atomic_int32_cas=no
 mandrill      | 2020-06-05 09:20:03 | pgac_cv_gcc_atomic_int32_cas=no
 prairiedog    | 2020-06-05 09:55:49 | pgac_cv_gcc_atomic_int32_cas=no

It looks to me like this is mostly about compiler support not the
hardware; that doesn't make it not a problem, though.  (I also
remain skeptical about the quality of the compiler intrinsics
on non-mainstream hardware.)

			regards, tom lane