| From: | Andres Freund <andres(at)anarazel(dot)de> |
|---|---|
| To: | Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com> |
| Cc: | Matthias van de Meent <boekewurm+postgres(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
| Subject: | Re: POC: Unlocked path for GetSnapshotDataReuse |
| Date: | 2026-10-09 12:54:04 |
| Message-ID: | rpf3opp3uexuz37erhq6fif662xob3t7ewbxjkjifkirubloaq@vjwseeum7kns |
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| Lists: | pgsql-hackers |
Hi,
On 2026-10-09 05:57:52 +0000, Bertrand Drouvot wrote:
> + pg_memory_barrier();
> +
> + completions = pg_atomic_read_u64(&TransamVariables->xactCompletionCount);
>
> IIUC, the barriers order the surrounding memory accesses, but they do not guarantee
> that pg_atomic_read_u64() does not return an old value.
I do not believe that that is true. Except in the sense of allowing an update
to the atomic variable having happened between pg_memory_barrier() and the
pg_atomic_read_u64(), but that's something that is indistinguishable from the
increment happening happeng just after pg_atomic_read_membarrier_u64().
Greetings,
Andres Freund
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