Re: locked reads for atomics

On Fri, Feb 23, 2024 at 05:34:49PM -0800, Andres Freund wrote:
> On 2024-02-23 14:58:12 -0600, Nathan Bossart wrote:
>> +/*
>> + * pg_atomic_write_membarrier_u32 - write with barrier semantics.
>> + *
>> + * The write is guaranteed to succeed as a whole, i.e., it's not possible to
>> + * observe a partial write for any reader. Note that this correctly interacts
>> + * with both pg_atomic_compare_exchange_u32() and
>> + * pg_atomic_read_membarrier_u32(). While this may be less performant than
>> + * pg_atomic_write_u32() and pg_atomic_unlocked_write_u32(), it may be easier
>> + * to reason about correctness with this function in less performance-sensitive
>> + * code.
>> + *
>> + * Full barrier semantics.
>> + */
>
> The callout to pg_atomic_unlocked_write_u32() is wrong. The reason to use
> pg_atomic_unlocked_write_u32() is for variables where we do not ever want to
> fall back to spinlocks/semaphores, because the underlying variable isn't
> actually shared. In those cases using the other variants is a bug. The only
> use of pg_atomic_unlocked_write_u32() is temp-table buffers which share the
> data structure with the shared buffers case.

I removed the reference to pg_atomic_unlocked_write_u32().

--
Nathan Bossart
Amazon Web Services: https://aws.amazon.com