Re: Atomic operations within spinlocks

Andres Freund <andres(at)anarazel(dot)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