Re: locked reads for atomics

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: locked reads for atomics
Date: 2023-11-11 02:55:29
Message-ID: 20231111025529.GA1409540@nathanxps13
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On Fri, Nov 10, 2023 at 06:48:39PM -0800, Andres Freund wrote:
> Yes. We should optimize pg_atomic_exchange_u32() one of these days - it can be
> done *far* faster than a cmpxchg. When I was adding the atomic abstraction
> there was concern with utilizing too many different atomic instructions. I
> didn't really agree back then, but these days I really don't see a reason to
> not use a few more intrinsics.

I might give this a try, if for no other reason than it'd force me to
improve my mental model of this stuff. :)

>> It refers to "unlocked writes," but this isn't
>> pg_atomic_unlocked_write_u32_impl(). The original thread for this comment
>> [0] doesn't offer any hints, either. Does "unlocked" mean something
>> different here, such as "write without any barrier semantics?"
> It's just about not using the spinlock. If we were to *not* use a spinlock
> here, we'd break pg_atomic_compare_exchange_u32(), because the
> spinlock-implementation of pg_atomic_compare_exchange_u32() needs to actually
> be able to rely on no concurrent changes to the value to happen.

Thanks for clarifying. I thought it might've been hinting at something
beyond the compare/exchange implications.

Nathan Bossart
Amazon Web Services:

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