Re: valgrind versus pg_atomic_init()

Hi,

On 2020-06-14 18:55:27 -0700, Noah Misch wrote:
> Does something guarantee the write will be globally-visible by the time the
> first concurrent accessor shows up?

The function comments say:

*
* Has to be done before any concurrent usage..
*
* No barrier semantics.

> (If not, one could (a) do an unlocked ptr->value=0, then the atomic
> write, or (b) revert and improve the suppression.) I don't doubt it's
> fine for the ways PostgreSQL uses atomics today, which generally
> initialize an atomic before the concurrent-accessor processes even
> exist.

I think it's unlikely that there are cases where you could safely
initialize the atomic without needing some form of synchronization
before it can be used. If a barrier were needed, what'd guarantee the
concurrent access happened after the initialization in the first place?

Greetings,

Andres Freund